Modernize Python typing across the codebase

Automated via ruff UP006, UP007, UP035, UP045 rules (target: py311):

- Replace `typing.List`, `Dict`, `Tuple`, `Set`, `FrozenSet`, `Type`
  with their built-in equivalents (`list`, `dict`, `tuple`, etc.)
- Replace `typing.Optional[X]` with `X | None`
- Replace `typing.Union[X, Y]` with `X | Y`
- Move `Mapping`, `Sequence`, `Callable`, `Awaitable`,
  `MutableMapping`, `MutableSequence`, `Iterator`, `AsyncIterator`,
  `AsyncGenerator` imports from `typing` to `collections.abc`
- Remove now-unused `typing` imports
- Add `from __future__ import annotations` to 5 files that use
  forward-reference strings in `X | "Y"` annotations

examples/rag/rag-gemini.py

@@ -87,7 +87,7 @@ def get_rag_content():
     """Get the RAG content from the file."""
     script_dir = os.path.dirname(os.path.abspath(__file__))
     rag_content_path = os.path.join(script_dir, "assets", "rag-content.txt")
-    with open(rag_content_path, "r") as f:
+    with open(rag_content_path) as f:
         return f.read()