Add validateOptionExists flag to LookupField

[mstijak]

Summary

• Adds opt-in validateOptionExists flag to LookupField. When enabled and options is an array, the field reports invalidOptionText as a validation error if the stored selection isn't present in options — surfaces stale ids in ValidationGroup.
• Skipped when options isn't an array (server-driven onQuery/infinite lookups) so loading states don't false-positive.
• Default is false for back-compat.
• Adds 6 runtime tests, docs entries on the LookupField page, changelog note, and translations for all 7 standard locales (en-us, de-de, es-es, fr-fr, nl-nl, pt-pt, sr-latn-ba).

Closes #1271

Test plan

• yarn test — 514 passing
• tsc --noEmit clean
• Manual smoke: bind a stale id to a LookupField inside a ValidationGroup with validateOptionExists, confirm the error surfaces and clears after selecting a valid option