fix(create-lesson): strip stray ext_payload from core exercises on edit-mode reconstruction (#1919)

[astrapi69]

Description

Reopening a saved core-only lesson for editing failed the Review "Valid lesson structure" check with /steps/1/exercise/ext_payload must be object, even though the same lesson was fully green in the create flow immediately before saving. Save changes was blocked on a lesson the author never touched an extension in.

Closes #1919

Root cause (API/server mode only)

The GET lessons endpoint serves the lesson via response_model=Lesson with FastAPI's default exclude_none=False, so the Pydantic Exercise.ext_payload: dict | None = None serializes as ext_payload: null on every core exercise. Proven directly against the model:

Exercise(id='e1', type='matching', prompt='P').model_dump(mode='json')
  -> ext_payload present = True -> None

The lesson JSON-Schema types ext_payload as object-only (type: object, "Absent on core exercises") — while every other optional exercise field tolerates null via anyOf: [..., {type: null}]. So ext_payload is the single field that breaks. lessonToDraftInput copied the reloaded exercise verbatim into the wizard, and checkDraft → buildLessonFromDraft's ajv validation then rejected the null.

Dexie mode parses via the engine adapter (parseLesson), which omits the key entirely — verified a full generate → save → parse round-trip is clean — so this reproduced only in API mode.

Fix

reconstructExercise (in lessonToDraftInput) restores the invariant "ext_payload present iff the exercise is an ext: extension type": a core exercise carries no ext_payload; an extension exercise keeps its real payload. Mode-agnostic (a no-op in Dexie mode, since the key is already absent there). Scoped precisely to ext_payload — the broader regression check confirmed no other optional field breaks the schema the same way.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Testing

• make test slice passes locally: draft-to-lesson, lib/content/lesson/, lib/exercises/authoring/, CreateLesson, plus storage/content/ + lib/content/ sweep (77 files / 916 tests green); tsc --noEmit clean; eslint clean on changed files.
• TDD RED-first: 3 new tests in draft-to-lesson.test.ts reproduce the API-mode reload (ext_payload: null on core exercises) — before the fix they failed with the exact /steps/1/exercise/ext_payload must be object; after the fix they pass.
• Regression: an extension exercise's ext_payload is preserved through reconstruction (no over-correction), including in a mixed core+extension lesson.

Root cause additionally verified end-to-end at the source (backend Pydantic Exercise.model_dump emits ext_payload: None) and at the sink (Dexie engine parseLesson round-trip omits the key).

Documentation

• Manual test plan updated (DE + EN) with a Edit mode: reopened lesson fails 'Valid lesson structure' with ext_payload must be object #1919 regression step under "Edit a lesson (Edit an own (created/imported) lesson via the /create-lesson wizard #1740)".

Checklist

• Self-review performed
• Google-style docstring on the new helper explaining the WHY
• No new warnings or errors

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Generated by Claude Code