feat: add lifespan support to low-level MCP server

Adds a context manager based lifespan API in mcp.server.lowlevel.server to manage server lifecycles in a
type-safe way. This enables servers to:
- Initialize resources on startup and clean them up on shutdown
- Pass context data from startup to request handlers
- Support async startup/shutdown operations

src/mcp/server/lowlevel/server.py

@@ -68,7 +68,8 @@ import contextvars
 import logging
 import warnings
 from collections.abc import Awaitable, Callable
-from typing import Any, Sequence
+from contextlib import AbstractAsyncContextManager, asynccontextmanager
+from typing import Any, AsyncIterator, Generic, Sequence, TypeVar
 
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import AnyUrl
@@ -101,13 +102,36 @@ class NotificationOptions:
         self.tools_changed = tools_changed
 
 
-class Server:
+LifespanResultT = TypeVar("LifespanResultT")
+
+
+@asynccontextmanager
+async def lifespan(server: "Server") -> AsyncIterator[object]:
+    """Default lifespan context manager that does nothing.
+
+    Args:
+        server: The server instance this lifespan is managing
+
+    Returns:
+        An empty context object
+    """
+    yield {}
+
+
+class Server(Generic[LifespanResultT]):
     def __init__(
-        self, name: str, version: str | None = None, instructions: str | None = None
+        self,
+        name: str,
+        version: str | None = None,
+        instructions: str | None = None,
+        lifespan: Callable[
+            ["Server"], AbstractAsyncContextManager[LifespanResultT]
+        ] = lifespan,
     ):
         self.name = name
         self.version = version
         self.instructions = instructions
+        self.lifespan = lifespan
         self.request_handlers: dict[
             type, Callable[..., Awaitable[types.ServerResult]]
         ] = {
@@ -446,35 +470,43 @@ class Server:
         raise_exceptions: bool = False,
     ):
         with warnings.catch_warnings(record=True) as w:
-            async with ServerSession(
-                read_stream, write_stream, initialization_options
-            ) as session:
-                async for message in session.incoming_messages:
-                    logger.debug(f"Received message: {message}")
+            async with self.lifespan(self) as lifespan_context:
+                async with ServerSession(
+                    read_stream, write_stream, initialization_options
+                ) as session:
+                    async for message in session.incoming_messages:
+                        logger.debug(f"Received message: {message}")
 
-                    match message:
-                        case (
-                            RequestResponder(
-                                request=types.ClientRequest(root=req)
-                            ) as responder
-                        ):
-                            with responder:
-                                await self._handle_request(
-                                    message, req, session, raise_exceptions
-                                )
-                        case types.ClientNotification(root=notify):
-                            await self._handle_notification(notify)
+                        match message:
+                            case (
+                                RequestResponder(
+                                    request=types.ClientRequest(root=req)
+                                ) as responder
+                            ):
+                                with responder:
+                                    await self._handle_request(
+                                        message,
+                                        req,
+                                        session,
+                                        lifespan_context,
+                                        raise_exceptions,
+                                    )
+                            case types.ClientNotification(root=notify):
+                                await self._handle_notification(notify)
 
-                    for warning in w:
-                        logger.info(
-                            f"Warning: {warning.category.__name__}: {warning.message}"
-                        )
+                        for warning in w:
+                            logger.info(
+                                "Warning: %s: %s",
+                                warning.category.__name__,
+                                warning.message,
+                            )
 
     async def _handle_request(
         self,
         message: RequestResponder,
         req: Any,
         session: ServerSession,
+        lifespan_context: object,
         raise_exceptions: bool,
     ):
         logger.info(f"Processing request of type {type(req).__name__}")
@@ -491,6 +523,7 @@ class Server:
                         message.request_id,
                         message.request_meta,
                         session,
+                        lifespan_context,
                     )
                 )
                 response = await handler(req)