fix: attach MCP-created nodes/edges to a design (#225)

create_node/create_edge persisted rows with design_id=null when the
client omitted it (the MCP write tools), so they existed in the DB but
never rendered on the canvas until a container restart reconciled them.
Both routes now fall back to the first design, matching bulk-approve.

Also fix MCP resource reads (homelable://canvas, homelable://edges):
the framework passes a pydantic AnyUrl, not a str, which raised
"'AnyUrl' object has no attribute 'startswith'". Coerce to str.

EdgeResponse now exposes design_id for symmetry with NodeResponse.

ha-relevant: no

mcp/tests/test_resources.py

@@ -1,4 +1,5 @@
 import pytest
+from pydantic import AnyUrl
 from unittest.mock import AsyncMock, patch
 from app.resources import read_resource
 
@@ -45,3 +46,24 @@ async def test_read_scan_pending(mock_backend):
 async def test_read_unknown_uri(mock_backend):
     with pytest.raises(ValueError, match="Unknown resource URI"):
         await read_resource("homelable://unknown")
+
+
+@pytest.mark.anyio
+async def test_read_resource_accepts_anyurl(mock_backend):
+    # Regression for #225: the MCP framework calls the handler with a pydantic
+    # AnyUrl, not a str, which raised "'AnyUrl' object has no attribute
+    # 'startswith'". The handler must coerce to str and still route correctly.
+    await read_resource(AnyUrl("homelable://canvas"))
+    mock_backend.get.assert_called_once_with("/api/v1/canvas")
+
+
+@pytest.mark.anyio
+async def test_read_edges_anyurl(mock_backend):
+    await read_resource(AnyUrl("homelable://edges"))
+    mock_backend.get.assert_called_once_with("/api/v1/edges")
+
+
+@pytest.mark.anyio
+async def test_read_single_node_anyurl(mock_backend):
+    await read_resource(AnyUrl("homelable://nodes/abc123"))
+    mock_backend.get.assert_called_once_with("/api/v1/nodes/abc123")