test(negative_stat_cache): add integration tests for implicit directory negative stat cache

[alleaditya]

Description

This PR adds comprehensive integration test coverage for implicit directory negative stat cache behavior across disabled, finite, and infinite negative stat cache TTL configurations in GCSFuse.

Key Updates:

• Extended tools/integration_tests/negative_stat_cache with implicit directory negative stat cache tests.
• Fixed file path targets under --implicit-dirs=false to test negative file stat caching directly under testDirPath.
• Verified implicit directory listing, stat, and file open operations under various cache TTL settings.

Link to the issue in case of a bug fix.

https://b.corp.google.com/issues/539425086

Testing details

1. Manual - Executed go vet ./tools/integration_tests/negative_stat_cache/... and gofmt -s -w tools/integration_tests/negative_stat_cache/ to ensure clean code formatting and linting.
2. Unit tests - N/A
3. Integration tests - Executed full implicit_dir and negative_stat_cache integration test packages against GCS bucket gs://gcsfuse-e2e-m2-worker1-1784797719:
   • implicit_dir package: PASS (48/48 tests passed, 373.990s execution time)
   • negative_stat_cache package: PASS (all tests passed across disabled, finite, and infinite cache configs, 355.666s execution time)

Any backward incompatible change? If so, please explain.

N/A

[gemini-code-assist]

Code Review

This pull request introduces integration tests for negative stat cache behaviors (disabled, finite, and infinite) under various scenarios, including implicit directories, flat buckets with implicit directories disabled, and HNS folders. It also updates test_config.yaml to include test runs with the --implicit-dirs flag. The feedback recommends replacing assert.NoError with require.NoError before dereferencing returned pointers to avoid nil pointer dereference panics if a call fails. Additionally, it suggests refining the isImplicitDirsEnabled helper to correctly handle cases where --implicit-dirs=false is passed.