concore validate --format json returns wrong node_id for nodes without a label

[bhuvan-somisetty]

concore validate --format json reports the wrong node_id when a node has no label.

The warning is built as Node {node_id} has no label, but _build_entries() pulls the id back out with message[5:-9]. The suffix has no label is 13 chars, not 9, so part of it stays in the id.

Repro:

1. Make a workflow with one node that has no <y:NodeLabel>, e.g. <node id="n1"></node>
2. Run concore validate test.graphml --format json

Output:

"warnings": [
  {
    "message": "Node n1 has no label",
    "error_type": "missing_node_label",
    "node_id": "n1 has"
  }
],
...
"nodes_affected": ["n1 has"]

Expected: node_id and nodes_affected should be "n1".

Actual: they are "n1 has", so anything that uses the JSON output to find the node (editor, scripts) gets an id that doesn't exist.

The bad slice is at concore_cli/commands/validate.py line 94. Changing it to message[5:-len(" has no label")] fixes it, and a test for this case would be good too since no existing test checks node_id here.

Tested on dev at 32951ab, Python 3.12.10