feat: conformance test runner + protocol fixes

- conformance.py: declarative scenario runner (subset matcher, $kind/$gt/
  $enum/$required assertions, call_id/session_id echo checks), reference
  bridge, CLI entry (python -m aicc.conformance)
- bridge.py: protocol version check before parsing (protocol_mismatch),
  session expiry handling (session_expired), malformed tool_call ->
  invalid_input, emit_event() for async events
- client.py: Self return type
- in_process.py: drain bridge event queue after response (event delivery),
  Self return type
- websocket.py: Self return type, best-effort shutdown
- tests: 14 passing (incl. conformance scenarios, 9/9 core scenarios)
- ruff: all checks pass

aicc/tool.py

@@ -4,13 +4,13 @@ from __future__ import annotations
 
 import asyncio
 import inspect
-from dataclasses import dataclass, field
-from typing import Any, Awaitable, Callable
+from collections.abc import Awaitable, Callable
+from dataclasses import dataclass
+from typing import Any
 
 from aicc.protocol import ToolClass
 from aicc.schema import function_schema, return_schema
 
-
 ToolImpl = Callable[..., Any | Awaitable[Any]]