[MendixMau]

Closes #1786

Why

mxcli test --local --watch keeps one mxbuild --serve process alive for
the whole watch session and rebuilds through it on every change — a test
file edit or a model change. run --local --watch has had build-failure
recovery (restart the serve process, rebuild once more) for a while;
test --watch never had it wired in. On macOS (mxbuild 11.15.0), the
second build in a serve process fails on regenerated page/layout files,
and mxbuild reports that failure by resource key
(IO_PathNotFound_Path) rather than the English text the existing
recovery matches — so every model change after the first in a test --watch session reports ERROR the change could not be applied and the
loop never produces a verdict again.

What changed

• localapp.go: route test --watch rebuilds through serveRecovery
  (the same mechanism run --local --watch already uses), in headless
  mode.
• webclient_legacy_paths.go: recognize the resource-key form of the
  failure (IO_PathNotFound_Path) alongside the existing English-text
  match, including when the target directory exists but the specific file
  mxbuild wants does not.
• serve_recovery.go: a headless flag — nothing serves the client in a
  test run, so an incomplete web/dist after an otherwise-successful
  build is not a defect worth another restart+rebuild (that restart exists
  only for run --local --watch's browser-facing client).

How tested

Unit tests for the two previously-untested edges this change exercises:

• TestServeRecovery_HeadlessSkipsPrebuiltRestart
• TestLegacyClientBuildHintRecognizesMacOSKeyForm
• TestLegacyClientBuildHintMacOSKeyForm_DirExistsFileMissing

go test ./cmd/mxcli/docker/... -run 'TestServeRecovery_HeadlessSkipsPrebuiltRestart|TestLegacyClientBuildHintRecognizesMacOSKeyForm|TestLegacyClientBuildHintMacOSKeyForm_DirExistsFileMissing'
→ PASS (3/3)

Full cmd/mxcli/docker package suite: 2 failures, both pre-existing and
unrelated to this change — TestSyncJavaDependencies_ReturnsWhenOrphanedDaemonHoldsPipe
and TestSyncJavaDependencies_FailingMxStillFails, exercising the
orphan-pipe/Gradle-daemon watchdog in javadeps.go (a file this PR never
touches; diff is localapp.go, serve_recovery.go(+test),
webclient_legacy_paths.go(+test) only). Confirmed environment-only:
setsid is absent on this machine (the fake-mx test script needs it),
and the same test fails identically against vanilla origin/main at
f99080e93 with zero code from this PR applied (15.01s, same message).

Real-app repro and numbers

Against a copy of a real ~280MB Mendix app (~/mta-lab/w/w2test, a copy of
sl1, 13 tests across 2 .test.mdl files):

mxcli test tests/mdl/69-match-rules.test.mdl tests/mdl/120-slice1-behaviour-changes.test.mdl \
  --local --watch --db-type hsqldb --require-assertions -p App.mpr

step	time	result	load (1-min avg)
boot	86s	13/13 green	—
test-file edit, wrong @expect	23s	12/13 (1 red)	—
revert	22s	13/13 green	—
model bug (seeded ACT_Invoice_Pay pay-guard removal)	61s	12/13, test_5 red — a correct verdict	46-58
model revert	64s	13/13 green	—
broken model (CE1613, retrieve from a deleted entity)	21s	Errors: 1, Passed: 0 — never a pass	—

These are correctness numbers, not a speed claim: the machine was under
heavy concurrent load (other sessions) for this run. The property being
proven is that the model-bug step gets a verdict at all — before this
fix, that exact step errors with "could not be applied" every time on
macOS and the watch loop is stuck for the rest of the session.

Regression evidence

• Wrong @expect → red. Revert → green. (test-file-only loop, unaffected
  by this change, included as a control.)
• Seeded model bug → correct red verdict, not an error.
• Model revert → correct green verdict.
• Broken microflow (CE1613) → reported as an error, never a pass.
• Watch session shuts down cleanly on SIGINT and restores the project.

Known limitation

This recovers from the build failing in the prebuilt-client shape. If
mxbuild --serve itself dies (not just a failed build), test --watch
still needs a full session restart — out of scope here, which only wires
the existing run --local --watch recovery machinery into the path
test --watch never had it on.

Checklist

• Full-stack MDL: n/a (no new syntax)
• Test coverage: 3 new unit tests for previously-untested edges, plus
  real-app ABAB and 5 applicable negative controls
• Version compat: no change to generated MDL or project file format
• Security: no change to the test-endpoint's attack surface
• Scope: one concern per PR (watch-mode build recovery only)
• Documentation: behavior change is internal recovery, no user-facing
  flag or doc change needed

Marked as draft until CI confirms green (the 2 pre-existing, unrelated
docker package failures are documented above with cross-branch proof).

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