fix: silence _settings NotGiven leaks and tighten Google STT language method

Six pyright errors followed the same pattern: a value flowed out of
`self._settings.X` (typed `T | _NotGiven`) into a context that wanted
the plain `T`. Wrap each with `assert_given(...)` so the sentinel
gets stripped at the boundary:

- aws/nova_sonic/llm.py: `_settings.model` (in InvokeModel...Input)
  and `_settings.system_instruction` (passed to the adapter).
- deepgram/flux/base.py: iterating `_settings.keyterm`.
- google/stt.py: iterating `_settings.languages`.
- google/tts.py: iterating `_settings.speaker_configs`.
- openai/base_llm.py: `_settings.system_instruction` passed to the
  adapter.

Also takes a deeper pass at the related Google STT issue: the override
of `language_to_service_language` had been broadened to take
`Language | list[Language]` and return `str | list[str]`, a Liskov
violation against the base's `Language -> str | None` contract.
External callers always pass a single Language, and the only consumer
of the list path was Google STT's own `_get_language_codes`. Restore
the override to a single-Language signature and let
`_get_language_codes` iterate. The override is also tightened to
return `str` (narrower than the base's `str | None`, which is
LSP-compatible) since it always falls back to `"en-US"` rather than
returning None.

Net: -7 pyright errors (full-config run: 782 -> 775).

src/pipecat/services/aws/nova_sonic/llm.py

@@ -974,7 +974,9 @@ class AWSNovaSonicLLMService(LLMService[AWSNovaSonicLLMAdapter]):
     async def open_stream(self, client):
         """Open a bidirectional stream on the given client."""
         return await client.invoke_model_with_bidirectional_stream(
-            InvokeModelWithBidirectionalStreamOperationInput(model_id=self._settings.model)
+            InvokeModelWithBidirectionalStreamOperationInput(
+                model_id=assert_given(self._settings.model)
+            )
         )
 
     async def send_event(self, event_json: str, stream):
@@ -1127,7 +1129,7 @@ class AWSNovaSonicLLMService(LLMService[AWSNovaSonicLLMAdapter]):
             return None, []
         adapter = self.get_llm_adapter()
         llm_params = adapter.get_llm_invocation_params(
-            self._context, system_instruction=self._settings.system_instruction
+            self._context, system_instruction=assert_given(self._settings.system_instruction)
         )
         tools = (
             llm_params["tools"]

src/pipecat/services/deepgram/flux/base.py

@@ -253,7 +253,7 @@ class DeepgramFluxSTTBase(STTService):
             params.append(f"mip_opt_out={str(self._mip_opt_out).lower()}")
 
         # Add keyterm parameters (can have multiple)
-        for keyterm in self._settings.keyterm:
+        for keyterm in assert_given(self._settings.keyterm):
             params.append(urlencode({"keyterm": keyterm}))
 
         # Add tag parameters (can have multiple)

src/pipecat/services/google/stt.py

@@ -620,17 +620,20 @@ class GoogleSTTService(STTService):
         """
         return True
 
-    def language_to_service_language(self, language: Language | list[Language]) -> str | list[str]:
+    def language_to_service_language(self, language: Language) -> str:
-        """Convert Language enum(s) to Google STT language code(s).
+        """Convert a Language enum to a Google STT language code.
+
+        Narrower return type than the base class's ``str | None``: this
+        override always returns a string, falling back to ``"en-US"`` for
+        languages not in the verified mapping (see
+        :func:`language_to_google_stt_language`).
 
         Args:
-            language: Single Language enum or list of Language enums.
+            language: The Language enum value to convert.
 
         Returns:
-            str | List[str]: Google STT language code(s).
+            The Google STT language code.
         """
-        if isinstance(language, list):
-            return [language_to_google_stt_language(lang) or "en-US" for lang in language]
         return language_to_google_stt_language(language) or "en-US"
 
     def _get_language_codes(self) -> list[str]:
@@ -642,8 +645,9 @@ class GoogleSTTService(STTService):
         Returns:
             List[str]: Google STT language code strings.
         """
-        if self._settings.languages:
+        languages = assert_given(self._settings.languages)
-            return [self.language_to_service_language(lang) for lang in self._settings.languages]
+        if languages:
+            return [self.language_to_service_language(lang) for lang in languages]
         language_codes = assert_given(self._settings.language_codes)
         if language_codes:
             return list(language_codes)

src/pipecat/services/google/tts.py

@@ -1417,7 +1417,7 @@ class GeminiTTSService(GoogleBaseTTSService):
             if self._settings.multi_speaker and self._settings.speaker_configs:
                 # Multi-speaker mode
                 speaker_voice_configs = []
-                for speaker_config in self._settings.speaker_configs:
+                for speaker_config in assert_given(self._settings.speaker_configs):
                     speaker_voice_configs.append(
                         texttospeech_v1.MultispeakerPrebuiltVoice(
                             speaker_alias=speaker_config["speaker_alias"],

src/pipecat/services/openai/base_llm.py

@@ -39,7 +39,7 @@ from pipecat.processors.aggregators.llm_context import LLMContext
 from pipecat.processors.frame_processor import FrameDirection
 from pipecat.services.llm_service import FunctionCallFromLLM, LLMService
 from pipecat.services.settings import NOT_GIVEN as _NOT_GIVEN
-from pipecat.services.settings import LLMSettings, _NotGiven
+from pipecat.services.settings import LLMSettings, _NotGiven, assert_given
 from pipecat.utils.tracing.service_decorators import traced_llm
 
 
@@ -299,7 +299,7 @@ class BaseOpenAILLMService(LLMService[OpenAILLMAdapter]):
 
         params_from_context = adapter.get_llm_invocation_params(
             context,
-            system_instruction=self._settings.system_instruction,
+            system_instruction=assert_given(self._settings.system_instruction),
             convert_developer_to_user=not self.supports_developer_role,
         )