fix(fal): stabilize shark staging smoke tests

[cmlad]

Summary

• retry app health checks when the gateway returns a retryable startup failure via x-fal-needs-retry
• add regression coverage for retryable vs non-retryable app health check responses
• define a shark_smoke integration subset for the post-deploy shark workflow

Testing

• pytest -q projects/fal/tests/unit/test_app.py -k app_spawn_info_wait
• pytest --collect-only -q -m shark_smoke projects/fal/tests/integration
• pre-commit run --all-files

Refs INFRA-2576.

[linear]

INFRA-2576 Fix staging/shark integration tests