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-image.py

@@ -67,14 +67,14 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     logger.info(f"Starting bot")
 
     stt = GoogleSTTService(
-        credentials=os.getenv("GOOGLE_TEST_CREDENTIALS"),
+        credentials=os.environ["GOOGLE_TEST_CREDENTIALS"],
         settings=GoogleSTTService.Settings(
             languages=[Language.EN_US],
         ),
     )
 
     tts = GoogleTTSService(
-        credentials=os.getenv("GOOGLE_TEST_CREDENTIALS"),
+        credentials=os.environ["GOOGLE_TEST_CREDENTIALS"],
         settings=GoogleTTSService.Settings(
             voice="en-US-Chirp3-HD-Charon",
             language=Language.EN_US,
@@ -82,7 +82,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     )
 
     llm = GoogleLLMService(
-        api_key=os.getenv("GOOGLE_API_KEY"),
+        api_key=os.environ["GOOGLE_API_KEY"],
         settings=GoogleLLMService.Settings(
             model="gemini-2.5-flash-image",
             # model="gemini-3-pro-image-preview", # A more powerful model, but slower,