feat(gemini): change default modality to AUDIO

Modify the default modality in the `InputParams` class from TEXT to AUDIO
to better align with the intended use case for GeminiMultimodalLive
service.

src/pipecat/services/gemini_multimodal_live/gemini.py

@@ -146,7 +146,7 @@ class InputParams(BaseModel):
     top_k: Optional[int] = Field(default=None, ge=0)
     top_p: Optional[float] = Field(default=None, ge=0.0, le=1.0)
     modalities: Optional[GeminiMultimodalModalities] = Field(
-        default=GeminiMultimodalModalities.TEXT
+        default=GeminiMultimodalModalities.AUDIO
     )
     extra: Optional[Dict[str, Any]] = Field(default_factory=dict)