fix: scope AJV instances per lintDocument call to fix concurrent validation

[vlindhol]

Summary

The global AJV singleton in packages/core/src/rules/ajv.ts captured a resolve closure from whichever document first created the instance. When consumers like openapi-typescript process multiple APIs concurrently via Promise.all (using @redocly/openapi-core's lintDocument), the second+ document's resolve was silently discarded. This produced spurious warnings:

⚠ Example validation errored: can't resolve reference #/components/schemas/Foo
  from id .../spec.yaml#/paths/~1items/post/requestBody/content/application~1json/schema.

These warnings appeared for valid same-document $refs that have an example/examples sibling, but only when:

• Multiple APIs are processed concurrently (e.g. Promise.all over lintDocument)
• The specs contain cross-file $refs (triggering AJV's loadSchemaSync)

The redocly lint CLI was unaffected because it processes APIs sequentially.

I did a similar PR (#2135) last year to fix the same thing but it got stale, and this time I managed to create a minimal test case to prove the bug exists.

Fix

• Replace the global singleton with an AjvValidator class that encapsulates AJV instance caching
• Each lintDocument call creates its own AjvValidator instance, threaded through WalkContext → UserContext
• No more global state, no more stale closures

Changes

• packages/core/src/rules/ajv.ts — Refactored from free functions + global singleton to an AjvValidator class with a validate() method and private getAjv()/getValidator() methods
• packages/core/src/walk.ts — Added ajvValidator: AjvValidator to WalkContext and UserContext, threaded through all visitor context construction sites
• packages/core/src/lint.ts — Each lintDocument/lintConfig call creates new AjvValidator() on the context (also removes the old releaseAjvInstance() call and its associated FIXME)
• packages/core/src/rules/utils.ts — validateExample calls ctx.ajvValidator.validate() instead of the old free function

Test plan

• Added integration test: concurrent lint() via Promise.all on two specs with same-doc $ref + example + cross-file $ref — confirmed it fails before the fix and passes after
• All 853 existing tests pass (127 test files)
• Pre-commit hooks (lint + format) pass

[changeset-bot]

🦋 Changeset detected

Latest commit: 1157188

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@redocly/openapi-core	Patch
@redocly/cli	Patch
@redocly/respect-core	Patch

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