Issue #3591779: Exclude content_translation_source/outdated from PropSourceSuggester suggestions

Closes #3591779

What this MR does

PropSourceSuggester::isConsideredIrrelevant() filters out entity base fields that are never meaningful to offer as prop sources (default_langcode, revision_default, revision_log_message).

When one of a content entity type's bundles is translatable, content_translation adds two fixed-name bookkeeping base fields to the entity type: content_translation_source and content_translation_outdated. These were being offered as prop source suggestions. This adds both to the exclusion list, mirroring the sibling heuristic already present in ApiUiContentEntityReferenceControllers::isConsideredIrrelevant().

In practice only content_translation_outdated (a boolean) surfaced today; content_translation_source is a language field that the suggester does not match to any prop shape. It is excluded defensively, for parity with the sibling method.

Testing instructions

• PropSourceSuggesterTest::testTranslationMetadataFieldsAreConsideredIrrelevant passes.
• Reverting the two field names in isConsideredIrrelevant() makes that test fail on content_translation_outdated.
• The rest of PropSourceSuggesterTest still passes (adding content_translation/language to the module list does not disturb the existing provider cases).

AI usage disclosure

Per Drupal.org's policy on AI: the code change, the added test, and this description were drafted with Claude (Claude Code) and reviewed by a human contributor before pushing.