CSV Coverage Pass

Goal

Add a tiny, high-value non-React pass for @platejs/csv around the only two worthwhile seams:

• deserializeCsv.ts
• CsvPlugin.ts

Constraints

• Fast-lane only.
• No broad package sweep beyond these two files.
• Prefer real editor and plugin contracts over mocked implementation detail.

Slice

1. Add direct deserializeCsv coverage for:
   • default header mode
   • explicit array mode
   • invalid tiny CSV rejection
   • error-tolerance handling
2. Add a thin CsvPlugin contract spec for default options, bound CSV API, and parser metadata.
3. Allow one runtime fix if the direct tests expose a real bug.

Notes

• There were no existing csv package specs.
• Direct runtime probing already showed a real default-header bug: valid CSV with header: true returns undefined.