refactor: slash-and-burn Annotatable BuiltIn XBlock

[irfanuddinahmad]

Summary

Removes the built-in Annotatable XBlock and all associated assets. The extracted xblocks_contrib.annotatable.AnnotatableBlock has been the active implementation since USE_EXTRACTED_ANNOTATABLE_BLOCK = True.

Step 3 of 8 in the Slash-n-Burn Built-In XBlocks epic.

Closes openedx/public-engineering#535

Note: xmodule/annotator_mixin.py is intentionally kept — it is still used by cms/djangoapps/contentstore/courseware_index.py and xmodule/util/misc.py for the html_to_text utility function and is not part of this removal.

Changes

• Remove USE_EXTRACTED_ANNOTATABLE_BLOCK toggle from openedx/envs/common.py
• Remove conflicting built-in annotatable entrypoint from pyproject.toml (the xblocks-contrib entrypoint remains active)
• Delete xmodule/annotatable_block.py
• Delete xmodule/js/src/annotatable/display.js, JS spec, and HTML fixture
• Delete lms/templates/annotatable.html
• Delete xmodule/static/css-builtin-blocks/AnnotatableBlock{Display,Editor}.css
• Remove AnnotatableBlockDisplay / AnnotatableBlockEditor webpack entries
• Delete xmodule/tests/test_annotatable_block.py
• Update stale AnnotatableBlock reference in xblock_serializer/utils.py docstring

Epic Step 1: Post-extraction commit review

Reviewed commits to xmodule/annotatable_block.py after USE_EXTRACTED_ANNOTATABLE_BLOCK was set to True (2025-12-16 · 24cb3cf9f5). All 4 post-toggle commits were cosmetic: deprecation markers (#38174), ruff suppression, import sorting, and lint-amnesty removal. No functional bug fixes or features to port.

Test plan

Testing scope: The xblocks-contrib implementation's behavioral correctness was fully validated during the extraction phase (when USE_EXTRACTED_* was set to True). These slash-n-burn PRs only delete dead built-in code — no behavioral change is possible. The only regression risk is that an entrypoint, webpack bundle, or import path is broken, which surfaces as a block failing to load. Basic load verification across LMS, CMS course, and CMS library is sufficient.

Smoke tests require a live Tutor environment and cannot be pre-checked in CI. Please verify during staging.

• Block loads and renders in LMS (student view)
• Block loads and renders in CMS course unit
• Block loads and renders in CMS V2 Library

Automated checks (CI):

• Run xmodule unit tests: pytest xmodule/tests/
• Confirm webpack build succeeds: npm run build
• Confirm annotator_mixin.html_to_text still works (used by search indexing)

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