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/common/server.env

@@ -42,7 +42,7 @@ OXICLOUD_NEXTCLOUD_ENABLED=true
 # `s3_stub` is declared but never activated — it lets storage_config.hurl
 # assert the entries table has more than one row without needing a
 # real S3 backend.
-OXICLOUD_STORAGE_ENTRIES=local_main,s3_stub,azurite
+OXICLOUD_STORAGE_ENTRIES=local_main,s3_stub,azurite,s3_blackhole
 OXICLOUD_STORAGE_local_main_BACKEND=local
 OXICLOUD_STORAGE_s3_stub_BACKEND=s3
 OXICLOUD_STORAGE_s3_stub_S3_BUCKET=oxicloud-test-stub
@@ -51,6 +51,36 @@ OXICLOUD_STORAGE_s3_stub_S3_ENDPOINT_URL=http://127.0.0.1:9999
 OXICLOUD_STORAGE_s3_stub_S3_ACCESS_KEY=stub
 OXICLOUD_STORAGE_s3_stub_S3_SECRET_KEY=stub
 
+# `s3_blackhole` — an endpoint that never answers, as opposed to
+# `s3_stub` above which refuses instantly.
+#
+# The distinction is the entire point. `127.0.0.1:9999` has nothing
+# listening, so a connection is REFUSED: the kernel returns ECONNREFUSED
+# immediately and the SDK reports an error straight away. That path was
+# always handled. A test built on it would pass even with no timeout
+# configured anywhere.
+#
+# `192.0.2.1` is TEST-NET-1 (RFC 5737), reserved for documentation and
+# guaranteed not to be routed. A SYN to it goes unanswered — no RST, no
+# ICMP — which is the failure that used to hang: with no bound, a socket
+# read blocks until the OS abandons retransmission, on the order of
+# fifteen minutes, during which the job is neither running nor failed.
+#
+# Declared but NEVER activated, like the other two. `backend_migration
+# ?storage=s3_blackhole` reaches it explicitly.
+#
+# If a CI network answers 192.0.2.1 with ICMP unreachable, the failure
+# degrades to the refused shape and `backend_migration_blackhole.hurl`
+# still passes — both classify transient and both pause. It would simply
+# stop pinning the timeout specifically. The assert on elapsed time in
+# that file is what would notice.
+OXICLOUD_STORAGE_s3_blackhole_BACKEND=s3
+OXICLOUD_STORAGE_s3_blackhole_S3_BUCKET=oxicloud-blackhole
+OXICLOUD_STORAGE_s3_blackhole_S3_REGION=us-east-1
+OXICLOUD_STORAGE_s3_blackhole_S3_ENDPOINT_URL=http://192.0.2.1:9999
+OXICLOUD_STORAGE_s3_blackhole_S3_ACCESS_KEY=blackhole
+OXICLOUD_STORAGE_s3_blackhole_S3_SECRET_KEY=blackhole
+
 # `azurite` — a REAL, reachable Azure backend, unlike `s3_stub` above.
 # It points at the Azurite emulator started by spawn-db.sh, which speaks
 # the actual Blob REST API, so this is the only way to exercise the Azure