refactor: move MCP lifecycle out of AgentLoop (#5343)

tests/agent/test_loop_save_turn.py

@@ -1519,13 +1519,11 @@ async def test_run_agent_loop_goal_continue_message_reads_latest_metadata(
 @pytest.mark.asyncio
 async def test_process_direct_rejects_reserved_system_channel(tmp_path: Path) -> None:
     loop = _make_full_loop(tmp_path)
-    loop._connect_mcp = AsyncMock()  # type: ignore[method-assign]
     loop._process_message = AsyncMock(return_value=None)  # type: ignore[method-assign]
 
     with pytest.raises(ValueError, match="reserved for internal messages"):
         await loop.process_direct("external input", channel="system")
 
-    loop._connect_mcp.assert_not_awaited()
     loop._process_message.assert_not_awaited()
 
 
@@ -1534,7 +1532,6 @@ async def test_process_direct_skip_user_persist_does_not_save_retry_user(
     tmp_path: Path,
 ) -> None:
     loop = _make_full_loop(tmp_path)
-    loop._connect_mcp = AsyncMock()
     session = loop.sessions.get_or_create("api:default")
     session.add_message("user", "hello")
     session.add_message("assistant", "previous empty-response attempt")