Draft: feat: add content-entity-types endpoint for entity reference selection UI

Christian López Espínolarequested to merge
issue/canvas-3585354:3585354-backend-apis-content-entity-reference into 1.x

[Description of changes]

AI-Generated: Yes (Used Claude (Opus 4.7) to create plan / code).

Testing instructions

  • First step for someone to test the changes in this MR

Closes #3585354

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