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.

pyrightconfig.json

@@ -13,10 +13,7 @@
     "src/pipecat/pipeline",
     "src/pipecat/runner"
   ],
-  "exclude": [
-    "**/*_pb2.py",
-    "**/__pycache__"
-  ],
+  "exclude": ["**/*_pb2.py", "**/__pycache__"],
   "ignore": [
     "src/pipecat/adapters",
     "src/pipecat/audio",
@@ -28,7 +25,6 @@
     "src/pipecat/transports",
     "src/pipecat/utils",
     "tests",
-    "examples",
     "scripts",
     "docs"
   ],