🐛 Support additionalProperties in schema validation

[w3lld1]

Summary

• I added support for the standard JSON Schema additionalProperties keyword in local, selected, and nested schema validation.
• I included the keyword in runtime type validation and the generated configuration schema.
• I documented when to use additionalProperties versus unevaluatedProperties and added regression coverage.

Validation

• pytest -q --ignore=tests/benchmarks -m 'not jstest' tests (1,026 passed, 5 skipped, 2 deselected)
• pytest -q tests/schema (181 passed, 2 skipped)
• sphinx-build -nW --keep-going -b html docs/ /tmp/sphinx-needs-docs-1723
• ruff check .
• ruff format --check .
• targeted mypy check for the changed schema modules
• git diff --check origin/master...HEAD

Fixes #1723