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-gladia.py

@@ -54,9 +54,12 @@ transport_params = {
 async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     logger.info(f"Starting bot")
 
+    region = os.getenv("GLADIA_REGION", "us-west")
+    assert region in ("us-west", "eu-west"), f"Invalid GLADIA_REGION: {region}"
+
     stt = GladiaSTTService(
-        api_key=os.getenv("GLADIA_API_KEY", ""),
-        region=os.getenv("GLADIA_REGION"),
+        api_key=os.environ["GLADIA_API_KEY"],
+        region=region,
         settings=GladiaSTTService.Settings(
             language_config=LanguageConfig(
                 languages=[Language.EN],