Translate and strictly negotiate /related/ resources

Problem/Motivation

Currently the /related/ endpoints (/jsonapi/node/{id}/{field}) only pass their target entities through the swapped access checker. That leaves two gaps:

No strict negotiation. A related target with no translation in an accepted language silently falls back to the untranslated entity, instead of a strict miss. The client asked for a language it accepts; it receives one it did not, unlabelled.

Language cache poisoning. The related response does not declare the response-level headers (Accept-Language cache context). Because of that, a cold no-selector response is stored by the Dynamic Page Cache without language variance and served to a later Accept-Language request for the same URL.

Steps to reproduce

A translatable content type with a to-one entity reference field; source node A references target C, where C has en + fr translations (no de). Uninstall the Internal Page Cache to isolate this, leaving the Dynamic Page Cache on.

1. GET /jsonapi/node/{A}/{field} with Accept-Language: fr.
   Expected: C in fr.
   Actual: C in en (fallback / a cached default).
2. GET … with Accept-Language: de.
   Expected: 406 Not Acceptable.
   Actual: C in en (silent fallback).
3. GET … with ?langCode=de.
   Expected: 404 Not Found.
   Actual: C in en.
4. GET … with no header (warms the cache), then GET … with Accept-Language: fr.
   Actual: the second request is served the cached default (en).

Proposed resolution