Ref #3191588: Move plugin cache clear from preSave to postSave

Andrey Postnikovrequested to merge
issue/config_help-3433302:3433302-automated-drupal-11 into 3.0.x

Clear plugin manager cached definitions after the entity is persisted, not before. This follows core patterns (BlockContent, Menu, LayoutBuilderEntityViewDisplay) and prevents a race where definitions could be rebuilt and cached without the new entity.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

Closes #3433302

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