fix(rest): change SSE SetWriteDeadline error log to debug level

[wuqinqiang]

#5155
#5094

When using rest.WithSSE() with timeout middleware enabled and clients not sending "Accept: text/event-stream" header,
so
https://github.com/zeromicro/go-zero/blob/master/rest/handler/timeouthandler.go#L63

SetWriteDeadline fails on timeoutWriter which doesn't implement the SetWriteDeadline interface.

This error is expected behavior and doesn't affect SSE functionality:

• SSE still works correctly without clearing write deadline
• timeoutWriter wraps ResponseWriter when timeout middleware is active
• Error occurs when clients don't set proper Accept header

The error should be logged at debug level instead of error level to
avoid confusing users who see "feature not supported" errors that
don't actually indicate a problem.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[Copilot]

Pull Request Overview

This PR fixes a logging issue in the Server-Sent Events (SSE) functionality where an expected error was being logged at error level instead of debug level. When timeout middleware is enabled and clients don't send the proper Accept header, the SetWriteDeadline method fails on timeoutWriter implementations, which is expected behavior that doesn't affect SSE functionality.

• Changes error log to debug level for SetWriteDeadline failures in SSE connections
• Adds explanatory comment about why this error is expected and harmless
• Improves user experience by reducing confusing error messages

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}