Add comprehensive test coverage for remote MCP server health check failures

[amirejaz]

Summary

This PR adds comprehensive test coverage for remote MCP server health check failure scenarios, ensuring that the health check callback mechanism works correctly when remote servers become unavailable.

Changes

• Added 8 new test cases covering various remote MCP server failure scenarios:
  • TestTransparentProxy_RemoteServerFailure_5xxStatus - Tests 5xx status codes trigger callback
  • TestTransparentProxy_RemoteServerFailure_ConnectionRefused - Tests connection failures
  • TestTransparentProxy_RemoteServerFailure_Timeout - Tests timeout scenarios
  • TestTransparentProxy_RemoteServerFailure_BecomesUnavailable - Tests server becoming unavailable after being healthy
  • TestTransparentProxy_RemoteServerFailure_Different5xxCodes - Table-driven test for various 5xx codes (500, 502, 503, 504)
  • TestTransparentProxy_RemoteServerHealthy_4xxCodes - Verifies 4xx codes are considered healthy
  • TestTransparentProxy_HealthCheckNotRunBeforeInitialization - Ensures health checks skip until initialization
  • TestTransparentProxy_HealthCheckFailureWithNilCallback - Tests graceful handling with nil callback

Technical Improvements

• Fixed race conditions by converting IsServerInitialized from int32 to atomic.Bool for thread-safe access
• Added private serverInitialized() getter method for better encapsulation
• All tests run in parallel and pass with race detector enabled

Testing

• All tests pass with -race flag enabled
• Tests use httptest.NewServer to simulate different failure scenarios
• Proper synchronization with sync.Mutex for callback tracking

Related

Addresses test coverage gap identified in PR #3077 for remote MCP server health check failure handling.

[codecov]

Codecov Report

❌ Patch coverage is 75.53191% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.78%. Comparing base (92b08b7) to head (896b161).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...g/transport/proxy/transparent/transparent_proxy.go	77.52%	12 Missing and 8 partials ⚠️
pkg/transport/proxy/transparent/pinger.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3084      +/-   ##
==========================================
+ Coverage   63.66%   63.78%   +0.11%     
==========================================
  Files         362      362              
  Lines       35521    35263     -258     
==========================================
- Hits        22613    22491     -122     
+ Misses      11089    10954     -135     
+ Partials     1819     1818       -1     

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[jhrozek]

Good test coverage for the health check failure scenarios!