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/persistent-context/persistent-context-gemini.py

@@ -110,7 +110,7 @@ async def load_conversation(params: FunctionCallParams):
     filename = params.arguments["filename"]
     logger.debug(f"loading conversation from {filename}")
     try:
-        with open(filename, "r") as file:
+        with open(filename) as file:
             params.context.set_messages(json.load(file))
         await params.result_callback(
             {