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/client.py

@@ -4,11 +4,16 @@ from __future__ import annotations
 
 import asyncio
 import uuid
-from typing import Any
+from typing import Any, Self
 
+from aicc.errors import (
+    AICCError,
+    ConnectionError_,
+    ProtocolError,
+    ToolError,
+)
 from aicc.protocol import (
     PROTOCOL_VERSION,
-    ErrorCode,
     EventMessage,
     MessageType,
     SessionClose,
@@ -16,12 +21,6 @@ from aicc.protocol import (
     ToolCall,
     ToolResult,
 )
-from aicc.errors import (
-    AICCError,
-    ConnectionError_,
-    ProtocolError,
-    ToolError,
-)
 from aicc.transport.base import Transport
 
 
@@ -56,14 +55,14 @@ class AICCClient:
 
     # ---------- Lifecycle ----------
 
-    async def __aenter__(self) -> "AICCClient":
+    async def __aenter__(self) -> Self:
         await self._transport.connect()
         return self
 
     async def __aexit__(self, exc_type, exc, tb) -> None:
         await self.close()
 
-    async def connect(self) -> "AICCClient":
+    async def connect(self) -> AICCClient:
         await self._transport.connect()
         return self