Enhance WebSocket upgrade detection in Mihomo

- Updated the `isWebSocketUpgrade` function to improve handling of WebSocket upgrade requests by considering the presence of the "Sec-WebSocket-Key" header as a strong indicator.
- Added a new test case in `TestIsWebSocketUpgrade` to validate the detection logic for requests with the "Sec-WebSocket-Key" header, ensuring comprehensive test coverage.

internal/proxy/mihomo_test.go

@@ -53,4 +53,10 @@ func TestIsWebSocketUpgrade(t *testing.T) {
 	if isWebSocketUpgrade(r2) {
 		t.Fatal("expected non-websocket when Connection lacks upgrade")
 	}
+
+	r3 := httptest.NewRequest(http.MethodGet, "http://gw/api/mtg/mihomo/traffic", nil)
+	r3.Header.Set("Sec-WebSocket-Key", "dGhlIHNhbXBsZSBub25jZQ==")
+	if !isWebSocketUpgrade(r3) {
+		t.Fatal("expected websocket upgrade when Sec-WebSocket-Key is present")
+	}
 }