fix: make sandbox uid configurable across all languages

[usnavy13]

Summary

• replace mixed hardcoded sandbox UIDs with one shared SANDBOX_UID override for all sandbox languages
• default all sandbox languages to non-root UID 1001, including Python, Java, and D
• update docs and tests to reflect the shared configurable sandbox UID model

Validation

• python3 -m flake8 src/ --count --select=E9,F63,F7,F82 --show-source --statistics
• python3 -m flake8 src/ --count --exit-zero --max-complexity=10 --statistics
• python3 -m black src/ --check
• python3 -m mypy src/
• .venv/bin/python -m bandit -r src/ -s B104,B108 --severity-level high
• pytest -q tests/unit/
• docker build -t code-interpreter:ci-check .
• API_BASE='http://localhost:8080' API_KEY='<repo .env key>' API_TIMEOUT=120 pytest -q tests/functional

Notes

• The second flake8 command is the same exit-zero warning sweep used by CI; it reported existing repository style warnings but does not fail the workflow.
• The live functional suite passed end-to-end with SANDBOX_UID=2001 set in the running host .env.