Fixed false update state for API feature image captions

[aileen]

Summary

• Prevented the feature image caption editor from writing Lexical hydration-only markup back to featureImageCaption when opening API-created posts.
• Added an acceptance regression test proving the Update button remains disabled when a plain text feature image caption is loaded from the API.

Root cause

The nested caption editor normalizes plain text captions into Lexical-generated HTML on initialization. That initial normalization was written directly back to the Ember Data post model, marking published posts as dirty before any user edit.

Validation

• pnpm --dir ghost/admin exec eslint app/components/gh-editor-feature-image.js tests/acceptance/editor/feature-image-test.js
• pnpm --dir ghost/admin exec ember exam --split 1 --parallel=1 --filter='Acceptance: Feature Image'

closes ONC-1765
closes #28094

[coderabbitai]

Walkthrough

This PR adds caption HTML normalization and comparison logic to the feature image editor. New utility functions (cleanCaptionHtml, normalizeCaptionHtml, isLexicalPlainTextSpan) handle HTML parsing and unwrapping of Lexical plain-text SPAN elements. The setCaption action now normalizes both the new and current captions, compares them, and skips the update when the normalized content is equivalent. An acceptance test validates that API-provided feature image captions render correctly and the publish/save button remains disabled.

Possibly related issues

• Update button incorrectly enabled on load when feature_image_caption is set via Admin API #28094: The caption cleaning/normalization and early-return logic in setCaption directly addresses the issue where the editor incorrectly treats API-provided feature_image_caption as a different/dirty value.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Fixed false update state for API feature image captions' accurately describes the main change: preventing false dirty state when loading API-created posts with feature image captions.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description clearly relates to the changeset, explaining the root cause, changes made, and validation steps for fixing the feature image caption editor.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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