refactor(api): make tool_handlers require a DeviceManager

Eliminates the silent fallback to DEFAULT_MANAGER that produced the
DeviceNotFoundError incident. All existing callers already pass
manager explicitly.

tests/test_mcp.py

@@ -64,3 +64,12 @@ def test_mcp_ui_tree_can_include_active_app_info() -> None:
         "activity": ".MainActivity",
     }
     assert isinstance(response["nodes"], list)
+
+
+def test_tool_handlers_requires_manager() -> None:
+    """D12: tool_handlers must not silently fall back to DEFAULT_MANAGER."""
+    from api.mcp import tool_handlers
+    import pytest
+
+    with pytest.raises(TypeError):
+        tool_handlers()  # type: ignore[call-arg]