Clear stale incremental diagnostics after JSON module changes

[Copilot]

Incremental builds retained diagnostics from an earlier JSON module shape after the JSON file was fixed.

• Root cause: JSON modules used an empty declaration emit as their shape signature, so subsequent content changes appeared shape-equivalent.
• Change: Use the JSON file’s content version as its shape signature, invalidating dependent diagnostics on every content change.
• Coverage: Add a regression covering valid JSON, removal of a required property, and restoration of that property.

• Fixes --incremental: diagnostics caused by a JSON module are never cleared after the JSON file is fixed (7.0.2) #64025