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

tests/test_service_language.py

@@ -10,7 +10,8 @@ Verifies that Language enums, raw strings (e.g. "de-DE"), and unrecognized
 strings are all resolved correctly at both init time and runtime update time.
 """
 
-from typing import AsyncGenerator, Optional
+from collections.abc import AsyncGenerator
+from typing import Optional
 from unittest.mock import patch
 
 import pytest
@@ -45,7 +46,7 @@ class _TestTTSService(TTSService):
     async def run_tts(self, text: str, context_id: str) -> AsyncGenerator[Frame, None]:
         yield  # pragma: no cover
 
-    def language_to_service_language(self, language: Language) -> Optional[str]:
+    def language_to_service_language(self, language: Language) -> str | None:
         return resolve_language(language, _LANGUAGE_MAP, use_base_code=True)
 
 
@@ -64,7 +65,7 @@ class _TestSTTService(STTService):
     async def process_audio_frame(self, frame, direction):
         pass  # pragma: no cover
 
-    def language_to_service_language(self, language: Language) -> Optional[str]:
+    def language_to_service_language(self, language: Language) -> str | None:
         return resolve_language(language, _LANGUAGE_MAP, use_base_code=True)