Include examples in type checking

Remove `examples/` from the `pyrightconfig.json` ignore list and fix
the resulting type errors across all example files. Common fixes:

- Required API keys: `os.getenv("X")` -> `os.environ["X"]` so the
  return type is `str` rather than `str | None`, and misconfiguration
  fails fast.
- Narrow `LLMContextMessage` union members with `isinstance(..., dict)`
  before dict-style access.
- `assert isinstance(params.llm, ...)` before calling service-specific
  methods that aren't on the base `LLMService`.
- Guard optional frame fields (e.g. `LLMSearchResponseFrame.search_result`)
  before use.

examples/voice/voice-deepgram-sagemaker.py

@@ -58,8 +58,8 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     # - AWS credentials configured (via environment variables or AWS CLI)
     # - A deployed SageMaker endpoint with Deepgram model
     stt = DeepgramSageMakerSTTService(
-        endpoint_name=os.getenv("SAGEMAKER_STT_ENDPOINT_NAME"),
-        region=os.getenv("AWS_REGION"),
+        endpoint_name=os.environ["SAGEMAKER_STT_ENDPOINT_NAME"],
+        region=os.environ["AWS_REGION"],
     )
 
     # Initialize Deepgram SageMaker TTS Service
@@ -67,8 +67,8 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     # - AWS credentials configured (via environment variables or AWS CLI)
     # - A deployed SageMaker endpoint with Deepgram TTS model
     tts = DeepgramSageMakerTTSService(
-        endpoint_name=os.getenv("SAGEMAKER_TTS_ENDPOINT_NAME"),
-        region=os.getenv("AWS_REGION"),
+        endpoint_name=os.environ["SAGEMAKER_TTS_ENDPOINT_NAME"],
+        region=os.environ["AWS_REGION"],
         settings=DeepgramSageMakerTTSService.Settings(
             voice="aura-2-andromeda-en",
         ),