Add JSON Schema validation to JsonInput via AJV

[lbwexler]

Summary

Adds optional schema validation to JsonInput via AJV. Split out of #4116 (CodeMirror v6 migration) so each PR has a focused scope.

• New props on JsonInput:
  • ajvSchema — JSON Schema object passed to AJV for content validation
  • ajvOptions — options forwarded to the AJV constructor
• When ajvSchema is provided, validation errors are surfaced as CodeMirror lint annotations alongside the existing jsonlint syntax checks. Error highlights point at the offending property in the source text.
• When ajvSchema is omitted, behavior is unchanged from the v6 baseline (jsonlint-only).
• New direct dep: ajv ~8.17.1.

Known limitations / follow-ups

• The AJV validator is currently rebuilt on every JsonInput render (new Ajv() + ajv.compile()). Consumers passing a stable schema reference still pay this cost. Worth memoizing.
• Error positioning uses text.indexOf(\"key"`)`, which finds the first occurrence of the property name. If the same key appears in multiple objects, the highlight can land on the wrong one.

Both are acceptable for a first cut but worth tracking.

Test plan

• npx tsc --noEmit clean
• yarn lint clean
• Manual smoke: JsonInput without ajvSchema lints syntax errors as before
• Manual smoke: JsonInput with an ajvSchema flags schema violations (missing required props, type mismatches, additionalProperties: false)

🤖 Generated with Claude Code

[lbwexler]

this is work done originally by Ryan and Pete --- I just split it off from the (orthogonal) CodeMirror upgrade it was bundled with