Validate JSON products too

[nikki-t]

P1-15 — Validate JSON products too.

• Let a contract declare a .json product (setfinder/combine_data definitions, coordination files).
• Route it through the JsonResult reader from P1-4 so it gets the same existence, shape, and required checks as NetCDF.

AC:

• A contract with a .json product validates that file the same way a .nc product does.

[nikki-t]

This may be put on hold as the only current JSON products are the input files that drive workflow execution and are not needed for the validation operations.

[nikki-t]

Tracked in beads as confluence-contracts-vzz (Phase 1, plan: #5). Sub-tasks and dependency state live there; this issue stays the team-visible record and closes when its PR merges.