Add minProperties to QueryGraph.nodes

[tokebe]

Proposed by @colleenXu in 2026-02-19 meeting, we don't want queries with no nodes to be valid

[colleenXu]

How I uncovered the issue:

• using Combine classes #544 as a starting point, made a json-schema of QueryGraph (plus all dependent subschemas)
• example QueryGraph to adjust and see what validates or doesn't
• plug into https://www.jsonschemavalidator.net/

The schema-validation succeeds when nodes is {} (empty)...but this leads to strange QueryGraphs that don't seem doable.

Like {nodes: {}} or the example above with an edge and path but no nodes.