feat: 406 advertises available translations; collection cache-variance spec+test

Lauri Timmaneerequested to merge
issue/jsonapi_multilingual-3612568:3612568-406-responses-should into 1.0.x
  • 406 Not Acceptable now lists the resource's available translations, both in the error detail and as machine-readable meta.availableTranslations (RFC 7231 §6.5.6), injected via ResponseSubscriber (JSON:API's exception normalizer cannot be extended).
  • collection-translation: add the no-selector cache-variance scenario and the behavioral testCollectionIsNotCachedAcrossLanguages (follow-up to #3612470).

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

Closes #3612568

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