Address feedback: cache git version, improve error handling, add telemetry

- Cache the git version to avoid recomputing on repeated calls
- Refactor getGitVersion to getGitVersionOrThrow with detailed errors
- Add getGitVersion that logs errors and handles caching
- Add makeTelemetryDiagnostic helper to diagnostics.ts
- Add logGitVersionTelemetry function to log git version telemetry
- Call logGitVersionTelemetry in init-action.ts
- Add resetCachedGitVersion for testing
- Update tests to work with new function signatures and caching

Co-authored-by: henrymercer <[email protected]>

Author: Copilot