Translation writes reject sparse payloads that omit the langcode attribute

Lauri Timmaneerequested to merge
issue/jsonapi_multilingual-3613674:3613674-translation-writes-reject into 1.0.x

The payload langcode is optional per the module's own contract ("if present, MUST match"): checkResourceLanguage() now skips the comparison when the body carries no langcode attribute, since the target language is unambiguous from the request metadata. The mismatch rejection still fires verbatim whenever a langcode attribute is present and disagrees. Sparse POST/PATCH coverage proves creation, isolation of the targeted translation, and the preserved 422.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

Closes #3613674

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