test: add unit tests of the app (#305)

* test: add unit tests in server

* test: add unit tests of app.py in server

* test: reformat the codes

* test: add more tests to cover the exception part

* test: add more tests on the server app part

* fix: don't show the detail exception to the client

* test: try to fix the CI test

* fix: keep the TTS API call without exposure information

* Fixed the unit test errors

* Fixed the lint error

tests/integration/test_tts.py

@@ -244,5 +244,6 @@ class TestVolcengineTTS:
         result = tts.text_to_speech("Hello, world!")
         # Verify the result
         assert result["success"] is False
-        assert result["error"] == "Network error"
+        # The TTS error is caught and returned as a string
+        assert result["error"] == "TTS API call error"
         assert result["audio_data"] is None