Add assertion for session_manager not None (pyright)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wiggzz

tests/server/test_streamable_http_manager.py

@@ -314,6 +314,7 @@ async def handle_call_tool(ctx: ServerRequestContext, params: Any) -> CallToolRe
         httpx.ASGITransport(mcp_app) as transport,
     ):
         session_manager = app._session_manager
+        assert session_manager is not None
 
         async def make_and_abandon_tool_call():
             async with httpx.AsyncClient(transport=transport, base_url=f"http://{host}", timeout=30.0) as http_client: