Improves error handling for canceled requests

Adds explicit handling for context.Canceled errors in the reverse proxy error handler to return 499 status code without logging, which is more appropriate for client-side cancellations during polling.

Also adds a test case to verify this behavior.

internal/api/modules/amp/proxy.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"compress/gzip"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -191,7 +192,6 @@ func createReverseProxy(upstreamURL string, secretSource SecretSource) (*httputi
 	proxy.ErrorHandler = func(rw http.ResponseWriter, req *http.Request, err error) {
 		// Client-side cancellations are common during polling; suppress logging in this case
 		if errors.Is(err, context.Canceled) {
-			rw.WriteHeader(gin.StatusClientClosedRequest)
 			return
 		}
 		log.Errorf("amp upstream proxy error for %s %s: %v", req.Method, req.URL.Path, err)