Fix #355: Fix type error with lifespan context (#368)

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

tests/issues/test_355_type_error.py

@@ -0,0 +1,50 @@
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
+from dataclasses import dataclass
+
+from mcp.server.fastmcp import Context, FastMCP
+
+
+class Database:  # Replace with your actual DB type
+    @classmethod
+    async def connect(cls):
+        return cls()
+
+    async def disconnect(self):
+        pass
+
+    def query(self):
+        return "Hello, World!"
+
+
+# Create a named server
+mcp = FastMCP("My App")
+
+
+@dataclass
+class AppContext:
+    db: Database
+
+
+@asynccontextmanager
+async def app_lifespan(server: FastMCP) -> AsyncIterator[AppContext]:
+    """Manage application lifecycle with type-safe context"""
+    # Initialize on startup
+    db = await Database.connect()
+    try:
+        yield AppContext(db=db)
+    finally:
+        # Cleanup on shutdown
+        await db.disconnect()
+
+
+# Pass lifespan to server
+mcp = FastMCP("My App", lifespan=app_lifespan)
+
+
+# Access type-safe lifespan context in tools
+@mcp.tool()
+def query_db(ctx: Context) -> str:
+    """Tool that uses initialized resources"""
+    db = ctx.request_context.lifespan_context.db
+    return db.query()