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_test.go

@@ -39,3 +39,18 @@ func TestMihomoForwardRewritesPathAndAuth(t *testing.T) {
 	}
 	assertSameURL(t, cap.got.URL, want)
 }
+
+func TestIsWebSocketUpgrade(t *testing.T) {
+	r1 := httptest.NewRequest(http.MethodGet, "http://gw/api/mtg/mihomo/traffic", nil)
+	r1.Header.Set("Connection", "keep-alive, Upgrade")
+	r1.Header.Set("Upgrade", "websocket")
+	if !isWebSocketUpgrade(r1) {
+		t.Fatal("expected websocket upgrade for tokenized headers")
+	}
+
+	r2 := httptest.NewRequest(http.MethodGet, "http://gw/api/mtg/mihomo/traffic", nil)
+	r2.Header.Set("Upgrade", "websocket")
+	if isWebSocketUpgrade(r2) {
+		t.Fatal("expected non-websocket when Connection lacks upgrade")
+	}
+}