Add WebSocket upgrade handling and tests in Mihomo

- Implemented enhanced logic in `isWebSocketUpgrade` to accurately determine WebSocket upgrade requests by checking both "Connection" and "Upgrade" headers.
- Added a new test function `TestIsWebSocketUpgrade` to validate the WebSocket upgrade detection logic, ensuring correct behavior for various header configurations.

internal/proxy/mihomo.go

@@ -36,7 +36,21 @@ func isWebSocketUpgrade(r *http.Request) bool {
 	if r == nil {
 		return false
 	}
-	return strings.EqualFold(r.Header.Get("Upgrade"), "websocket")
+	// Some clients/proxies pass comma-separated tokens or extra spaces.
+	// Treat request as WS only when both headers contain required upgrade tokens.
+	return headerHasToken(r.Header, "Connection", "upgrade") &&
+		headerHasToken(r.Header, "Upgrade", "websocket")
+}
+
+func headerHasToken(h http.Header, key, token string) bool {
+	for _, v := range h.Values(key) {
+		for _, part := range strings.Split(v, ",") {
+			if strings.EqualFold(strings.TrimSpace(part), token) {
+				return true
+			}
+		}
+	}
+	return false
 }
 
 func newMihomoWebSocketReverseProxy(