refactor: remove built-in HTML XBlock implementation (slash-n-burn #37819)

[irfanuddinahmad]

Summary

Removes the built-in HTML XBlock from xmodule/html_block.py as part of the Slash-n-Burn Built-In XBlocks epic (#37819). The extracted version in xblocks-contrib has been the active implementation since USE_EXTRACTED_HTML_BLOCK was set to True.

Closes openedx/public-engineering#536

Deleted:

• _BuiltinHtmlBlockMixin class (~330 LOC) — built-in HTML mixin
• _BuiltInHtmlBlock class — built-in HTML block
• HtmlBlockMixin = None / reset_Mixin() — shim for the mixin
• HtmlBlock = None / reset_class() — shim for HtmlBlock
• xmodule/static/css-builtin-blocks/HtmlBlockDisplay.css
• All unused imports from html_block.py

Kept:

• AboutBlock, StaticTabBlock, CourseInfoBlock — still live in html_block.py; now inherit from xblocks_contrib.html.HtmlBlockMixin directly
• HtmlBlockEditor webpack entry — still loaded by the instructor dashboard
• xmodule/js/src/html/edit.js, HtmlBlockEditor.css — referenced by instructor dashboard template

Modified:

• openedx/envs/common.py — removed USE_EXTRACTED_HTML_BLOCK toggle
• pyproject.toml — removed conflicting html entrypoint
• webpack.builtinblocks.config.js — removed HtmlBlockDisplay entry
• xmodule/tests/test_html_block.py — removed toggle-conditional test classes; kept CourseInfoBlockTestCase
• lms/djangoapps/courseware/tests/test_block_render.py — import HtmlBlock from xblocks_contrib.html

Note: xmodule/js/src/html/display.js and imageModal.js are still referenced by AnnotatableBlockDisplay in webpack (removed in PR #38746). After both #38746 and this PR merge, those two JS files become dead code. Their deletion is tracked in #38792.

Epic Step 1: Post-extraction commit review

Reviewed commits to xmodule/html_block.py after USE_EXTRACTED_HTML_BLOCK was set to True (2025-12-03 · 1fed4be5c9). All 6 post-toggle commits were cosmetic or independently mirrored: the HtmlBlockMixin base refactor (b2376c5f7c, 2026-01-01) was independently applied in xblocks-contrib (b277504) with the same intent; remaining commits were 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
• Course About page renders in LMS — AboutBlock still lives in html_block.py, now inheriting from xblocks_contrib.html.HtmlBlockMixin
• Static Tabs render in LMS course navigation — same reason (StaticTabBlock)
• Course Info/Updates tab renders in LMS — same reason (CourseInfoBlock)
• HtmlBlockEditor (TinyMCE) loads in CMS course unit editor — the HtmlBlockEditor webpack bundle was intentionally retained

Automated checks (CI):

• Run pytest xmodule/tests/test_html_block.py — CourseInfoBlockTestCase should pass
• Run pytest lms/djangoapps/courseware/tests/test_block_render.py — no HtmlBlock import errors

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