Combobox Coverage Pass

Goal

Add a surgical, fast-lane-only non-React pass for the remaining worthwhile @platejs/combobox seam:

• uncovered branches in withTriggerCombobox.ts

Constraints

• Do not touch /react.
• Do not broaden filterWords just to chase coverage; it already has the right helper matrix.
• Keep this as one adjacent spec expansion, not a package rewrite.

Slice

1. Add withTriggerCombobox coverage for:
   • regex trigger matching with the default combobox node path
   • Yjs userId propagation onto the inserted input node
   • triggerQuery veto fallback
   • options.at fallback
2. Stop unless direct tests expose a real bug.

Notes

• Existing withTriggerCombobox coverage already covers array and string triggers plus previous-character matching.
• The remaining value is in the untested fallback and default-node branches.