🐛 Fixed false update state for API feature image captions (#28107)

closes [ONC-1765](https://linear.app/ghost/issue/ONC-1765/oss-issue-update-button-incorrectly-enabled-on-load-when-feature-image]
closes #28094

The feature image caption editor was writing Lexical hydration markup back to `featureImageCaption` when API-created posts with plain text captions were opened, which marked the post as dirty before any user edit. Normalizing generated plain-text spans before comparing caption HTML keeps the loaded record clean while preserving real caption edits, and the regression test verifies the `Update` button remains disabled on load.