feat: Refine get_file_contents tool with allow_raw_fallback parameter

[yonaka15]

This PR addresses the feedback from github/github-mcp-server#599.

Changes

• Added allow_raw_fallback parameter to get_file_contents tool:
  • This new boolean parameter allows controlling whether the tool can fall back to fetching raw file content.
  • When set to false, the tool will always use the Get Contents API endpoint, ensuring that the file's SHA is included in the response.
  • The default behavior remains true to avoid breaking existing functionality for large files.
• Unified Error Handling:
  • Replaced instances of fmt.Errorf with mcp.NewToolResultError in the modified tool for consistent error reporting.
• Updated Unit Tests:
  • Added test cases to validate the behavior of the allow_raw_fallback parameter for both true and false scenarios.
  • Corrected mock handler usage to ensure tests pass reliably.

All unit tests pass successfully.