Fix potential goroutine leak in httpreqresp.Do function

[Copilot]

The Do function in cl/sentinel/httpreqresp/server.go had a potential goroutine leak due to an unbuffered channel. When the request context was cancelled before the HTTP handler completed, the goroutine would block indefinitely on ans <- resp because no receiver was available.

Root Cause:

func Do(handler http.Handler, r *http.Request) (*http.Response, error) {
    ans := make(chan *http.Response)  // Unbuffered channel
    go func() {
        // ... handler processing ...
        ans <- resp  // Blocks forever if context cancelled first
    }()
    select {
    case res := <-ans:
        return res, nil
    case <-r.Context().Done():
        return nil, r.Context().Err()  // No one left to receive from ans
    }
}

Solution:
Changed the channel to be buffered with capacity 1:

ans := make(chan *http.Response, 1)

This allows the goroutine to send its response without blocking, even when the context cancellation case wins the race condition. The goroutine can complete normally and be garbage collected.

Testing:
Added comprehensive tests in server_test.go that verify:

• Context cancellation doesn't cause goroutine leaks
• Normal operation continues to work correctly
• No regression in existing functionality

The fix is minimal (adding , 1 to the channel declaration) and maintains backward compatibility while preventing resource leaks.

Fixes #16665.

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