feat: slash-and-burn Problem block built-in implementation

[irfanuddinahmad]

Summary

Removes the built-in ProblemBlock shim (xmodule/capa_block.py) and all associated assets, completing the extraction of the Problem XBlock to xblocks_contrib. Part of the Slash-n-Burn Built-In XBlocks epic.

• Deleted xmodule/capa_block.py (2481 LOC — _BuiltInProblemBlock class and shim)
• Deleted xmodule/capa/ (deprecated safe_exec shim pointing to xblocks_contrib)
• Deleted xmodule/js/src/capa/ JS files (display.js, imageinput.js, schematic.js)
• Deleted xmodule/static/css-builtin-blocks/ProblemBlockDisplay.css
• Deleted xmodule/tests/test_capa_block.py (directly tested _BuiltInProblemBlock)
• Deleted lms/djangoapps/grades/tests/integration/test_problems.py (relied on reset_class shim)
• Removed USE_EXTRACTED_PROBLEM_BLOCK Django setting toggle from openedx/envs/common.py
• Updated CODE_JAIL_REST_SERVICE_REMOTE_EXEC to point directly to xblocks_contrib.problem.capa.safe_exec.remote_exec
• Removed problem entrypoint from pyproject.toml
• Removed ProblemBlockDisplay webpack entry from webpack.builtinblocks.config.js
• Updated all ProblemBlock, SHOWANSWER, and generate_report_data imports/patches to use xblocks_contrib.problem

Test plan

• Run pytest xmodule/tests/test_delay_between_attempts.py — no capa_block imports
• Run pytest xmodule/tests/test_item_bank.py — ProblemBlock from xblocks_contrib
• Run pytest xmodule/tests/test_library_content.py — ProblemBlock from xblocks_contrib
• Run pytest cms/djangoapps/contentstore/tests/test_crud.py
• Run pytest cms/djangoapps/contentstore/tests/test_contentstore.py
• Run pytest lms/djangoapps/courseware/tests/test_block_render.py
• Run pytest lms/djangoapps/instructor_task/tests/test_tasks_helper.py
• Run pytest openedx/features/personalized_learner_schedules/show_answer/tests/
• Verify Problem XBlocks render correctly in LMS and CMS

🤖 Generated with Claude Code

Closes openedx/public-engineering#540

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