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 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.25%. Comparing base (4b3fd00) to head (44ca6c2).

Files with missing lines	Patch %	Lines
...g/transport/proxy/transparent/transparent_proxy.go	89.65%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3084      +/-   ##
==========================================
+ Coverage   57.14%   57.25%   +0.11%     
==========================================
  Files         341      341              
  Lines       34053    34075      +22     
==========================================
+ Hits        19459    19510      +51     
+ Misses      12985    12953      -32     
- Partials     1609     1612       +3     

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[jhrozek]

Good test coverage for the health check failure scenarios!