test(api): pin that an unreachable backend pauses in bounded time

The regression test `docs/plan/jobs-handling-recoverable-error.md`
§Testing asks for: assert the run reaches Paused, that `error_message`
names the cause, and that it does so in bounded time rather than
hanging.

## The endpoint has to HANG, not refuse

`s3_stub` already existed and points at `127.0.0.1:9999`, where nothing
listens. That connection is REFUSED — ECONNREFUSED, immediately — and
that path was never broken. A test built on it would pass with no
timeout configured anywhere, which is worse than no test: it would read
as coverage of exactly the failure it cannot see.

So `s3_blackhole` points at `192.0.2.1`, TEST-NET-1 (RFC 5737),
reserved for documentation and guaranteed unrouted. A SYN goes
unanswered — no RST, no ICMP — which is the failure that used to hang
until the OS abandoned TCP retransmission ~15 minutes later, with the
job neither running nor failed the whole time.

Ed's suggestion, and it is the right fixture: a server that never
answers is reproducible in a way that unplugging a cable is not.

## The load-bearing assertion is `duration`

Every other assert in the file would also pass against the old hanging
behaviour, given fifteen minutes. `duration < 120000` is the only one
that fails if the bound is ever removed. The threshold is deliberately
loose — three orders of magnitude from the failure it guards, so a slow
runner cannot make it flaky.

## Why it is safe in the shared suite

The run fails at `target.initialize()`, which is BEFORE
`migration_readonly` is engaged, so this file cannot leave the server
read-only for whatever runs next. A mid-copy failure would have held
the freeze — that is why this shape was chosen.

Teardown is mandatory rather than tidy: `open_or_start` picks up the
latest non-terminal row, so a Paused row left behind would be RESUMED
by the next `backend_migration` trigger in the suite, silently
retargeting an unrelated test at the black hole. The file cancels its
own run and asserts the row reached Cancelled.

Placed second-to-last. It is the slowest file in the suite by design —
it waits out an unreachable endpoint to prove the wait is bounded — so
that cost lands after everything else has reported. Azurite stays last
for the reason its own comment gives.

Not yet executed: the suite tears down containers and Ed usually has a
run in flight.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/api/backend_migration_blackhole.hurl

@@ -0,0 +1,165 @@
+# =============================================================
+# OxiCloud – backend_migration against an endpoint that never answers
+#
+# The regression test asked for by
+# `docs/plan/jobs-handling-recoverable-error.md` §Testing: assert that
+# an unreachable backend lands the run in **Paused**, that
+# `error_message` names the cause, and that it gets there in **bounded
+# time rather than hanging**.
+#
+# ## The failure this pins
+#
+# A backend that *fails* was always handled — classified, retried,
+# paused. A backend that never *answers* was handled by nothing. Pull a
+# network on an established connection and there is no RST and no ICMP;
+# the socket blocks until the OS abandons retransmission, on the order
+# of fifteen minutes. Throughout that window the job is neither running
+# nor failed: no error, so no retry, no log line, no pause. It looks
+# exactly like a slow migration.
+#
+# Worse, before the classification fixes the `blob_exists` source probe
+# reported such a failure as PERMANENT, which took the
+# record-a-finding-and-continue branch — the cursor advanced past the
+# blob, and with `failed` still 0 the run could reach `finish_completed`
+# and flip the pointer to a target missing everything the outage
+# covered. A migration reporting success having silently dropped
+# whatever was unreachable at the time.
+#
+# ## Why `s3_blackhole` and not `s3_stub`
+#
+# `s3_stub` points at `127.0.0.1:9999`, where nothing listens, so the
+# connection is REFUSED instantly. That path was never broken. A test
+# built on it would pass with no timeout configured anywhere and pin
+# nothing.
+#
+# `s3_blackhole` points at `192.0.2.1` — TEST-NET-1 (RFC 5737),
+# reserved for documentation and guaranteed unrouted. A SYN goes
+# unanswered, which is the hang. See `tests/common/server.env`.
+#
+# ## Why this is safe inside the shared suite
+#
+# The migration fails at `target.initialize()`, which runs BEFORE
+# `migration_readonly` is engaged (`backend_migration_service.rs`, the
+# comment on the target-init pause). So this file cannot leave the
+# server read-only for whatever runs after it — the reason this shape
+# was chosen over a mid-copy failure, which would hold the freeze.
+#
+# The run is cancelled at the end regardless, so the DB is left with no
+# non-terminal `backend_migration` row.
+#
+# Prerequisites: setup.hurl must have run (admin user exists).
+# =============================================================
+
+
+# ─────────────────────────────────────────────────────────────
+# Step 1 — Log in as admin.
+# ─────────────────────────────────────────────────────────────
+POST {{base_url}}/api/auth/login
+Content-Type: application/json
+{
+  "username": "{{username}}",
+  "password": "{{password}}"
+}
+
+HTTP 200
+[Captures]
+admin_token: jsonpath "$.access_token"
+
+
+# ─────────────────────────────────────────────────────────────
+# Step 2 — Trigger the migration at the black hole.
+#
+# The trigger is synchronous, so the response IS the outcome.
+#
+# `outcome.outcome == "ok"` is deliberate and not a contradiction: a
+# retryable pause is carried as `Ok` because the handler did its job and
+# stopped cleanly at a checkpoint. `extra.paused` / `extra.retryable`
+# are what distinguish it, which is exactly why the scheduler log line
+# had to stop projecting a paused run as a clean one.
+#
+# The `duration` assert is the heart of this file. Everything else here
+# would also pass against the old hanging behaviour — given fifteen
+# minutes. This is the only assertion that fails if the bound is ever
+# removed, so treat it as load-bearing rather than a performance nicety.
+#
+# 120s: comfortably above the observed ~31s (the SDK's own attempts
+# stacked on the 10s connect timeout) and far below the ~15min the
+# unbounded socket would take. Deliberately loose — a slow CI runner
+# must not make this flaky, and the failure it guards against is three
+# orders of magnitude away, not adjacent.
+# ─────────────────────────────────────────────────────────────
+POST {{base_url}}/api/admin/jobs/backend_migration/trigger?storage=s3_blackhole
+Authorization: Bearer {{admin_token}}
+
+HTTP 200
+[Captures]
+blackhole_run_id: jsonpath "$.outcome.extra.run_id"
+[Asserts]
+duration < 120000
+jsonpath "$.ok" == true
+jsonpath "$.outcome.outcome" == "ok"
+jsonpath "$.outcome.extra.paused" == true
+jsonpath "$.outcome.extra.retryable" == true
+jsonpath "$.outcome.extra.run_id" exists
+# The reason must name what went wrong, not merely that something did.
+# An operator reading only this string has to be able to tell an
+# unreachable backend from a wrong bucket — the first is worth waiting
+# out, the second never resolves on its own.
+jsonpath "$.outcome.extra.reason" contains "Transient Backend"
+jsonpath "$.outcome.extra.reason" contains "target backend init"
+
+
+# ─────────────────────────────────────────────────────────────
+# Step 3 — The run row must agree with the outcome.
+#
+# `Paused`, not `Failed`: the distinction is the whole plan. Failed is
+# terminal and needs a human to decide what happened; Paused resumes
+# and finishes the migration once the backend returns.
+#
+# `completed_at` must be absent — the run is not over. A paused row
+# carrying a completion timestamp would make every "how long did this
+# take" query lie, and would read as finished in the admin panel.
+# ─────────────────────────────────────────────────────────────
+GET {{base_url}}/api/admin/jobs/backend_migration/runs/{{blackhole_run_id}}
+Authorization: Bearer {{admin_token}}
+
+HTTP 200
+[Asserts]
+jsonpath "$.status" == "Paused"
+jsonpath "$.error_message" exists
+jsonpath "$.error_message" contains "Transient Backend"
+jsonpath "$.completed_at" not exists
+
+
+# ─────────────────────────────────────────────────────────────
+# Step 4 — Teardown: cancel the paused run.
+#
+# Mandatory, not tidiness. `open_or_start` picks up the latest
+# non-terminal row for a job name, so a `Paused` row left behind would
+# be RESUMED by the next `backend_migration` trigger in the suite —
+# silently retargeting that run at the black hole and failing a test
+# that has nothing to do with this file. Hurl files share one database.
+#
+# Cancel is also the path that releases `migration_readonly` for a
+# paused row (nothing to release here — this run never engaged it —
+# but the call is idempotent).
+# ─────────────────────────────────────────────────────────────
+POST {{base_url}}/api/admin/jobs/backend_migration/cancel
+Authorization: Bearer {{admin_token}}
+
+HTTP 200
+[Asserts]
+jsonpath "$.cancelled" == true
+jsonpath "$.run_id" == "{{blackhole_run_id}}"
+
+
+# ─────────────────────────────────────────────────────────────
+# Step 5 — Confirm the row is terminal, so the next trigger in the
+# suite starts fresh instead of resuming ours.
+# ─────────────────────────────────────────────────────────────
+GET {{base_url}}/api/admin/jobs/backend_migration/runs/{{blackhole_run_id}}
+Authorization: Bearer {{admin_token}}
+
+HTTP 200
+[Asserts]
+jsonpath "$.status" == "Cancelled"