feat: migrate codejail sandbox to its own uv project (3/5)

[irfanuddinahmad]

Summary

PR 3 of 5 in the pip-compile -> uv migration tracked in openedx/public-engineering#543.

Stacked on #38836 (PR 2), which is itself stacked on #38835 (PR 1) — this diff only shows PR 3's own changes. It'll retarget automatically as each parent merges.

1. PR 1 (feat: add pyproject.toml deps, dependency-groups, and uv.lock (1/5) #38835) — populate pyproject.toml deps/dependency-groups + commit uv.lock
2. PR 2 (feat: cut over Makefile, tox.ini, and CI to uv (2/5) #38836) — cut over Makefile, tox.ini, and CI to uv for the main app
3. PR 3 (this PR) — migrate the codejail sandbox to its own standalone uv project
4. PR 4 — migrate the three standalone scripts to standalone uv projects
5. PR 5 — cleanup docs/README and follow-through on tracking issues

What changed

requirements/edx-sandbox/ (codejail's isolated sandbox environment for running untrusted instructor-authored Python) gets its own pyproject.toml + uv.lock, kept fully independent of the main app's dependency graph — codejail intentionally runs in a separate venv for security isolation, so this isn't merged into the root project's dependencies.

• [tool.uv].package = false — this was never pip install -e'd, just resolved into an isolated venv.
• [tool.edx_lint].uv_constraints holds only the subset of the root pyproject.toml's constraints actually relevant to this environment's dependencies (numpy<2.0.0, lxml==5.3.2, setuptools<82). uv/edx-lint have no equivalent to pip-compile's -c ../constraints.txt cross-project chaining, so root and sandbox constraints are now maintained independently — documented as an explicit trade-off in requirements/edx-sandbox/README.rst.
• base.in deleted; base.txt is now a uv export --no-hashes --no-emit-project compatibility artifact (the README already documented this file as a supported, if unstable, direct pip install -r target — no reason to break that contract).
• releases/*.txt untouched — confirmed these are frozen historical snapshots, not part of any active compile loop (not in the old REQ_FILES list either). README now documents cutting future release snapshots via uv export ... -o releases/<name>.txt instead of pip-compile.
• Root Makefile: added a UV_SUBPROJECTS loop (currently just requirements/edx-sandbox) to compile-requirements/upgrade, running edx_lint write_uv_constraints + uv lock + regenerating the base.txt compat export for each sub-project.

External compatibility (tutor/Docker, org-level .github workflows)

• Tutor/Docker: confirmed via direct grep of the installed tutor plugin's Dockerfile (tutor/env/build/openedx/Dockerfile) that it does not reference requirements/edx-sandbox/ at all — only requirements/edx/{base,assets,development}.txt (handled in PR 2). So this PR's base.txt compat export isn't preserving a tutor/Docker contract; it's preserving the contract requirements/edx-sandbox/README.rst already documents for anyone directly pip install -r-ing this file outside of tutor.
• Org-level .github reusable workflows: none of this repo's workflows delegate sandbox-specific logic to openedx/.github, so there's nothing to reconcile there for this PR (see PR 2's body for the one org-level workflow that is relevant to this migration, upgrade-python-requirements.yml, which already supports uv).

Verification

• uv lock inside an ubuntu:24.04 container — resolves cleanly to 32 packages; confirmed constraints honored (numpy==1.26.4 not 2.x, lxml==5.3.2 exact match).
• uv sync --frozen + uv run python3 -c "import chem, cryptography, lxml, matplotlib, networkx, nltk, scipy, sympy, pyparsing" — all imports succeed.
• Full make compile-requirements re-run end-to-end (root uv lock + sandbox uv lock via the new UV_SUBPROJECTS loop + the still-pip-compiled scripts/*) — completes cleanly; sandbox's constraint-writing step is confirmed idempotent (re-run produces the same 5 merged constraints).

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