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-sarvam-http.py

@@ -58,14 +58,14 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     # Create an HTTP session
     async with aiohttp.ClientSession() as session:
         stt = SarvamSTTService(
-            api_key=os.getenv("SARVAM_API_KEY"),
+            api_key=os.environ["SARVAM_API_KEY"],
             settings=SarvamSTTService.Settings(
                 model="saarika:v2.5",
             ),
         )
 
         tts = SarvamHttpTTSService(
-            api_key=os.getenv("SARVAM_API_KEY"),
+            api_key=os.environ["SARVAM_API_KEY"],
             aiohttp_session=session,
             settings=SarvamHttpTTSService.Settings(
                 language=Language.EN_IN,
@@ -73,7 +73,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
         )
 
         llm = SarvamLLMService(
-            api_key=os.getenv("SARVAM_API_KEY"),
+            api_key=os.environ["SARVAM_API_KEY"],
             settings=SarvamLLMService.Settings(
                 system_instruction="You are a helpful assistant in a voice conversation. Your responses will be spoken aloud, so avoid emojis, bullet points, or other formatting that can't be spoken. Respond to what the user said in a creative, helpful, and brief way.",
             ),