fix: prevent superfluous WriteHeader error in SSE streaming

[sicko7947]

Added trackingResponseWriter wrapper to prevent the 'superfluous response.WriteHeader call' error that occurs when:

1. SSE streaming starts (WriteHeader is called)
2. An error occurs during streaming
3. Error handler tries to call WriteHeader again via http.Error()

The fix wraps http.ResponseWriter to track if headers have been written and gracefully handles errors that occur after streaming has started.

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[gemini-code-assist]

Summary of Changes

Hello @sicko7947, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a common issue in Go's net/http package where attempting to write response headers after they have already been sent can lead to runtime errors, particularly problematic in long-lived connections or streaming contexts like SSE. By introducing a wrapper around http.ResponseWriter that tracks header status, the change ensures that WriteHeader calls are idempotent and error handling is robust, preventing crashes and improving the stability of HTTP responses.

Highlights

• Introduced trackingResponseWriter: A new trackingResponseWriter struct has been added, which wraps the standard http.ResponseWriter to keep track of whether response headers have already been written.
• Prevented superfluous WriteHeader calls: The trackingResponseWriter explicitly checks if headers are already written before calling WriteHeader, logging a message and skipping the call if they are, thereby preventing the 'superfluous response.WriteHeader call' error, especially in streaming scenarios like Server-Sent Events (SSE).
• Improved error handling post-streaming: Error handling in EncodeJSONResponse and NewErrorHandler has been updated to gracefully manage errors that occur after response headers have been sent. If headers are already written, subsequent error responses (which would attempt to write new headers) are logged instead of causing a panic or error.

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[gemini-code-assist]

Code Review

This pull request effectively addresses the 'superfluous WriteHeader' error during SSE streaming by introducing a trackingResponseWriter. The approach is solid and the implementation is clean. My main feedback concerns a potential race condition in the implementation of the new response writer wrapper, which could occur with concurrent writes. I've suggested using the sync/atomic package to ensure thread safety. I also have a minor suggestion regarding the use of the standard log package for better configurability in production environments.