fix(cartesia): guard HTTP session before use

Pyright flagged the .post() call on a possibly-None _session. Raise a
clear RuntimeError if start() wasn't called instead of crashing on the
attribute access.

pyrightconfig.json

@@ -42,7 +42,6 @@
     "src/pipecat/services/azure/stt.py",
     "src/pipecat/services/azure/tts.py",
     "src/pipecat/services/cartesia/stt.py",
-    "src/pipecat/services/cartesia/tts.py",
     "src/pipecat/services/deepgram/flux/base.py",
     "src/pipecat/services/deepgram/flux/sagemaker/stt.py",
     "src/pipecat/services/deepgram/flux/stt.py",

src/pipecat/services/cartesia/tts.py

@@ -916,6 +916,9 @@ class CartesiaHttpTTSService(TTSService):
         logger.debug(f"{self}: Generating TTS [{text}]")
 
         try:
+            if self._session is None:
+                raise RuntimeError("HTTP session is not initialized; call start() before run_tts()")
+
             voice_config = {"mode": "id", "id": self._settings.voice}
 
             output_format = {