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-google.py

@@ -59,7 +59,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
             # Add model to use a specific model
             # model="chirp_3",
         ),
-        credentials=os.getenv("GOOGLE_TEST_CREDENTIALS"),
+        credentials=os.environ["GOOGLE_TEST_CREDENTIALS"],
         location="us",
     )
 
@@ -68,11 +68,11 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
             voice="en-US-Chirp3-HD-Charon",
             language=Language.EN_US,
         ),
-        credentials=os.getenv("GOOGLE_TEST_CREDENTIALS"),
+        credentials=os.environ["GOOGLE_TEST_CREDENTIALS"],
     )
 
     llm = GoogleLLMService(
-        api_key=os.getenv("GOOGLE_API_KEY"),
+        api_key=os.environ["GOOGLE_API_KEY"],
         settings=GoogleLLMService.Settings(
             model="gemini-2.5-flash",
             # force a certain amount of thinking if you want it