fix(gcp): allow getting project ID from credentials

[dmitsh]

No description provided.

[greptile-apps]

Greptile Summary

Refactored GCP project ID determination from runtime lookup to initialization-time resolution with a clear fallback chain.

Key Changes:

• Moved project ID lookup to provider initialization instead of per-API-call
• Implemented priority order: request params → GOOGLE_APPLICATION_CREDENTIALS file → metadata API
• Changed Client.ProjectID() interface from error-returning to simple string getter
• Added comprehensive unit tests covering all scenarios and error paths
• Documented the project ID override mechanism for users

Benefits:

• Fail-fast error handling at startup rather than during API calls
• Eliminates redundant metadata lookups on every request
• Provides users flexibility to override project ID via request parameters
• Maintains backward compatibility with existing deployments

Confidence Score: 5/5

• This PR is safe to merge with no concerns
• The refactoring is well-designed with comprehensive test coverage. All changes are backward compatible, error handling is robust, and the new initialization-time approach is more efficient and maintainable than the previous runtime lookup pattern
• No files require special attention

Important Files Changed

Filename	Overview
pkg/providers/gcp/instance_topology.go	Simplified to use client.ProjectID() directly, removing error-prone runtime lookups
pkg/providers/gcp/provider.go	Refactored project ID determination to initialization with proper fallback chain
pkg/providers/gcp/provider_test.go	New comprehensive unit tests covering all getProjectID scenarios and error paths

_{Last reviewed commit: b915dc7}

[greptile-apps]

_{4 files reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[codecov]

Codecov Report

❌ Patch coverage is 75.86207% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.91%. Comparing base (0fb6f58) to head (b915dc7).

Files with missing lines	Patch %	Lines
pkg/providers/gcp/provider.go	69.56%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
+ Coverage   65.90%   65.91%   +0.01%     
==========================================
  Files          82       82              
  Lines        4520     4536      +16     
==========================================
+ Hits         2979     2990      +11     
- Misses       1423     1428       +5     
  Partials      118      118              

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[greptile-apps]

Additional Comments (2)

docs/providers/gcp.md, line 85
parameter name uses hyphen but code expects underscore

      "project_id": "your-project-id"

---

pkg/providers/gcp/provider.go, line 113
error message says "is not a file" but the actual condition is "does not exist" - consider rewording for accuracy

			return "", fmt.Errorf("GOOGLE_APPLICATION_CREDENTIALS file does not exist: %v", err)