Drop AbstractAsyncContextManager for proper type hints (#257)

Author: Kludex

Diff for: src/mcp/shared/session.py

@@ -8,6 +8,7 @@
 import httpx
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import BaseModel
+from typing_extensions import Self
 
 from mcp.shared.exceptions import McpError
 from mcp.types import (
@@ -60,7 +61,7 @@ def __init__(
         request_id: RequestId,
         request_meta: RequestParams.Meta | None,
         request: ReceiveRequestT,
-        session: "BaseSession",
+        session: "BaseSession[SendRequestT, SendNotificationT, SendResultT, ReceiveRequestT, ReceiveNotificationT]",
         on_complete: Callable[["RequestResponder[ReceiveRequestT, SendResultT]"], Any],
     ) -> None:
         self.request_id = request_id
@@ -134,7 +135,6 @@ def cancelled(self) -> bool:
 
 
 class BaseSession(
-    AbstractAsyncContextManager,
     Generic[
         SendRequestT,
         SendNotificationT,
@@ -183,7 +183,7 @@ def __init__(
             ]()
         )
 
-    async def __aenter__(self):
+    async def __aenter__(self) -> Self:
         self._task_group = anyio.create_task_group()
         await self._task_group.__aenter__()
         self._task_group.start_soon(self._receive_loop)