Fix infinite save loop when editing a lesson with a quiz block

[donnapep]

Closes #7861

Description

When editing a Lesson that contains a Quiz block, the block editor could enter an infinite save loop and the Save button would spin on "Saving…" indefinitely. It reproduces when another active plugin injects extra fields into REST API responses — Polylang Pro does this via rest_pre_echo_response, adding lang and translations to every response, including Sensei's internal lesson-quiz endpoint.

Root cause

The quiz structure store compares the lesson-quiz REST response against the editor's structure with a deep-equality check (isEqual). The quiz store's setServerStructure() passed the response through with a ...structure spread, so any extra top-level field carried into the comparison. The injected lang/translations fields meant the comparison never matched, hasUnsavedServerUpdates stayed true, and finishPostSave() re-dispatched savePost() — repeating forever.

The course-outline store was already immune because its setServerStructure() rebuilds an explicit { structure } object rather than spreading the response.

Fix

Normalize the quiz response to the editor's known contract (lesson_status, lesson_title, options, questions) before it is stored and compared, extracted as a pure normalizeServerStructure() in assets/blocks/quiz/data.js. Any extra top-level fields are dropped, so the comparison reflects only what the editor actually tracks. This is resilient to fields injected by any plugin, not just Polylang, and mirrors the existing course-outline pattern.

Testing instructions

1. Activate a plugin that injects fields into REST responses (e.g. Polylang Pro), or simulate it with an mu-plugin that adds lang/translations on rest_pre_echo_response.
2. Create or open a Lesson, add a Quiz block with at least one question.
3. Change a question or quiz setting and click Save.
4. Confirm the lesson saves once — the editor no longer loops on "Saving…" and the Network tab shows a single POST to sensei-internal/v1/lesson-quiz/{id}.
5. With the injector disabled, confirm quizzes still save and load normally.

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Pull request overview

Fixes an infinite save loop in the Quiz block lesson editor flow by normalizing the lesson-quiz REST response to the editor’s expected structure contract, preventing injected REST fields (e.g. from Polylang) from causing perpetual deep-diff mismatches.

Changes:

• Normalizes quiz server structure before storing/comparing it in the quiz structure store.
• Extracts normalization logic into a pure normalizeServerStructure() helper and shares READ_ONLY_ATTRIBUTES via data.js.
• Adds unit tests to validate normalization behavior (including stripping injected top-level fields).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
changelog/fix-quiz-save-loop-injected-rest-fields	Adds a changelog entry documenting the fix.
assets/blocks/quiz/quiz-store.js	Switches quiz store server-structure normalization to normalizeServerStructure().
assets/blocks/quiz/data.js	Introduces normalizeServerStructure() and exports READ_ONLY_ATTRIBUTES.
assets/blocks/quiz/data.test.js	Adds unit tests covering normalization and injected-field stripping.

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