fix(ecs): surface failure reason on /logs when task fails to start

[vieiralucas]

Summary

Stabilize CI runner flakes (3/3) — make ECS task failures diagnosable.

The ECS secrets E2E test `ecs_task_resolves_secretsmanager_secret` has been failing intermittently in CI with `secret not injected: ` (note the empty string after the colon). Empty captured-logs means `finalize_failure` ran instead of `finalize_stopped` — i.e. the task died before the container started — but `finalize_failure` was not setting `task.captured_logs`, so the HTTP introspection endpoint `/_fakecloud/ecs/tasks/{id}/logs` returned an empty string and the assertion had nothing to attribute the failure to.

This PR is a diagnostic surface fix:

• `finalize_failure` (in `crates/fakecloud-ecs/src/runtime.rs`) now writes `[task failed to start]: ` into `task.captured_logs`, mirroring the assignment that `finalize_stopped` already performs on the success path.
• The `run_task` error handler also `eprintln!`s the reason so nextest's captured-output for a failed E2E surfaces it directly, not just inside the task state.
• Adds a unit test `finalize_failure_writes_reason_into_captured_logs` that asserts the reason is present in `captured_logs` and prefixed correctly.

The actual intermittency (real `resolve_secret` race vs disk-pressure side-effect of PRs #827/#828) is intentionally left untouched here; once those land and CI logs show the real reason text on a failure, we'll know which root cause to fix.

Sibling PRs: #827 (nextest retries + disk diagnostics), #828 (linker OOM via test profile debuginfo).

Test plan

• `cargo test -p fakecloud-ecs --lib finalize_failure` passes locally
• `cargo clippy -p fakecloud-ecs --all-targets -- -D warnings` clean
• CI runs green first try
• Next CI run that fails the ECS secrets test now contains the failure reason in the assertion message

---

Summary by cubic

Show ECS task start failure reasons on /_fakecloud/ecs/tasks/{id}/logs and stderr so failures no longer appear as empty logs.

• Bug Fixes
  • Set task.captured_logs in finalize_failure to "[task failed to start]: ".
  • Print the failure reason to stderr in run_task for clearer nextest output.
  • Add unit test finalize_failure_writes_reason_into_captured_logs.

^{Written for commit 2e33b98. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 1 file

[codecov]

Codecov Report

❌ Patch coverage is 98.48485% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/fakecloud-ecs/src/runtime.rs	98.48%	1 Missing ⚠️

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