fix(nc/webdav): MKCOL on a missing parent → 409 (RFC 4918 §9.3.1)

Closes F11.

The NC MKCOL handler previously had `mkdir -p` semantics:
sending MKCOL on /a/b/c/ where neither a nor b exists silently
created both intermediates and returned 201. Sabre/DAV and the
actual NC server both 409 on that — our auto-create deviated.
NC desktop walks ancestors one MKCOL at a time during sync so
nothing real depended on the old behaviour.

Drop the segment-walking creation loop. New flow:
  target exists           → 405
  parent path missing     → 409
  parent ok, target new   → 201

The race-recovery branch for the loop's per-segment create is
also gone — single parent lookup, single create, no window.

Test F11 flipped from pinned-201 to strict 409 and asserts the
intermediate parent was not silently created. F11b and F11c added
as regression guards for the success path and the 'target already
exists' case.

tests/webdav/test_nc_put_mkcol_blake3.sh

@@ -319,38 +319,43 @@ grep -q '<d:collection/>' <<< "$BODY" \
 pass "F10: MKCOL creates folder, PROPFIND sees it as a collection"
 
 # ─────────────────────────────────────────────────────────────
-# F11 — MKCOL with missing intermediate parent
+# F11 / F11b / F11c — MKCOL parent semantics (RFC 4918 §9.3.1)
 #
-# Pinned current behaviour: OxiCloud's MKCOL auto-creates
-# missing intermediate parents (effectively `mkdir -p`
-# semantics). Sending MKCOL on `/a/b/c/` where neither `a` nor
-# `b` exists succeeds with 201 — both intermediates are
-# silently created.
+# F11  : missing intermediate parent → 409 Conflict
+# F11b : parent exists, target new   → 201 Created (positive case)
+# F11c : target already exists       → 405 Method Not Allowed
 #
-# Strict RFC 4918 §9.3.1 requires 409 Conflict here ("when the
-# parent collection does not exist"). NC desktop tolerates
-# either behaviour (it always MKCOLs ancestors one at a time
-# during sync), so the auto-create behaviour is harmless in
-# practice — but if you ever want strict mode, the fix lives
-# in `interfaces/nextcloud/webdav_handler.rs::handle_mkcol`:
-# look up the parent path before creating; 409 if missing.
+# Sabre/DAV and the actual NC server both 409 on a missing
+# intermediate; our previous `mkdir -p` behaviour deviated. NC
+# desktop walks ancestors one MKCOL at a time during sync so
+# nothing real breaks from dropping the auto-create.
 # ─────────────────────────────────────────────────────────────
-echo "  F11: MKCOL with missing parent (pinned: auto-creates parents, RFC-4918 would 409)"
+echo "  F11: MKCOL with missing intermediate parent → 409"
 STATUS=$(nc_curl -o /dev/null -w "%{http_code}" -X MKCOL \
     "$NC_FILES_BASE/f11-nonexistent-parent/inner/")
-case "$STATUS" in
-    201)
-        pass "F11: MKCOL auto-created intermediate parents (201) — pinned current behaviour"
-        ;;
-    409)
-        fail "F11: server now returns 409 (RFC-4918 strict). Bug? Improvement? — review and update pin to strict assertion."
-        ;;
-    *)
-        fail "F11: unexpected status $STATUS"
-        ;;
-esac
-# Cleanup the auto-created parent so subsequent tests don't see it.
-nc_curl -o /dev/null -X DELETE "$NC_FILES_BASE/f11-nonexistent-parent/" > /dev/null 2>&1 || true
+[[ "$STATUS" == "409" ]] \
+    || fail "F11: expected 409 Conflict for MKCOL with missing parent, got $STATUS"
+# The non-existent parent must NOT have been auto-created either.
+[[ "$(nc_status_propfind_depth0 "$NC_FILES_BASE/f11-nonexistent-parent/")" == "404" ]] \
+    || fail "F11: intermediate parent was silently created — auto-create still happening"
+pass "F11: MKCOL with missing parent → 409, parent not silently created"
+
+echo "  F11b: MKCOL with existing parent + new target → 201"
+nc_curl -o /dev/null -X MKCOL "$NC_FILES_BASE/f11b-parent/" > /dev/null
+STATUS=$(nc_curl -o /dev/null -w "%{http_code}" -X MKCOL \
+    "$NC_FILES_BASE/f11b-parent/child/")
+[[ "$STATUS" == "201" ]] \
+    || fail "F11b: expected 201 Created for MKCOL with existing parent, got $STATUS"
+[[ "$(nc_status_propfind_depth0 "$NC_FILES_BASE/f11b-parent/child/")" == "207" ]] \
+    || fail "F11b: target collection not visible via PROPFIND after MKCOL"
+pass "F11b: MKCOL with existing parent → 201, target reachable"
+
+echo "  F11c: MKCOL with target that already exists → 405"
+STATUS=$(nc_curl -o /dev/null -w "%{http_code}" -X MKCOL \
+    "$NC_FILES_BASE/f11b-parent/child/")
+[[ "$STATUS" == "405" ]] \
+    || fail "F11c: expected 405 Method Not Allowed for MKCOL on existing collection, got $STATUS"
+pass "F11c: MKCOL on existing target → 405"
 
 # ─────────────────────────────────────────────────────────────
 # F12 — MKCOL on existing folder → 405