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-speechmatics-vad.py

@@ -91,7 +91,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     logger.info(f"Starting bot")
     async with aiohttp.ClientSession() as session:
         stt = SpeechmaticsSTTService(
-            api_key=os.getenv("SPEECHMATICS_API_KEY"),
+            api_key=os.environ["SPEECHMATICS_API_KEY"],
             settings=SpeechmaticsSTTService.Settings(
                 language=Language.EN,
                 turn_detection_mode=SpeechmaticsSTTService.TurnDetectionMode.ADAPTIVE,
@@ -102,7 +102,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
         )
 
         tts = SpeechmaticsTTSService(
-            api_key=os.getenv("SPEECHMATICS_API_KEY"),
+            api_key=os.environ["SPEECHMATICS_API_KEY"],
             settings=SpeechmaticsTTSService.Settings(
                 voice="sarah",
             ),
@@ -110,7 +110,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
         )
 
         llm = OpenAILLMService(
-            api_key=os.getenv("OPENAI_API_KEY"),
+            api_key=os.environ["OPENAI_API_KEY"],
             settings=OpenAILLMService.Settings(
                 temperature=0.75,
                 system_instruction="You are a helpful British assistant called Sarah in a voice conversation. Your responses will be spoken aloud, so avoid emojis, bullet points, or other formatting that can't be spoken. Always include punctuation in your responses. Give very short replies - do not give longer replies unless strictly necessary. Respond to what the user said in a concise, funny, creative and helpful way. Use `<Sn/>` tags to identify different speakers - do not use tags in your replies. Do not respond to speakers within `<PASSIVE/>` tags unless explicitly asked to.",