Add comprehensive test coverage for agent metadata update functionality

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Problem

The agent metadata update functionality introduced in PR #9285 lacks comprehensive test coverage. This creates a risk of regressions and makes it difficult to ensure the reliability of:

• Backward compatibility with existing function signatures
• New completion tracking features (isComplete and hasChanges)
• Diff stats collection and summary extraction
• Session usage tracking and error handling
• API client interactions and error resilience

Solution Needed

Implement comprehensive test coverage that validates:

• All success paths and error conditions
• Backward compatibility guarantees
• New feature functionality
• Graceful degradation when dependencies fail
• Proper mocking of external dependencies (git, API, session management)

Acceptance Criteria

• Test coverage for legacy function signature compatibility
• Test coverage for new isComplete and hasChanges functionality
• Test coverage for diff stats, summary extraction, and usage tracking
• Test coverage for error handling and graceful degradation
• Test coverage for API client error scenarios
• Documentation of testing strategy and principles
• All tests pass and provide confidence in reliability

This issue tracks the need for comprehensive testing to ensure the agent metadata functionality is robust and maintainable.

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Related issues:

• Improve agent metadata tracking with completion state and changes detection #9286 - Improve agent metadata tracking with completion state and changes detection (closed) - This is the issue that implemented the agent metadata update functionality that now needs test coverage