[envpool] Fix flexible lint toolchain drift

[Trinkle23897]

Summary

• fix the latest flexible lint drift surfaced by current tool versions instead of pinning older tooling
• make Bazel BUILD files compatible with current buildifier
• make addlicense, mypy, docstyle, and spelling pass again under current environments

Technical Details

• add explicit rules_python / rules_cc loads so current buildifier no longer flags legacy native rule usage
• add Apache headers to the affected third-party BUILD stubs and switch the generated header year to 2026
• tighten Python typing in the envpool adapters/tests so current mypy passes without pinning the checker
• remove the deprecated Sphinx theme-path usage and make the docs install path self-contained in clean environments by ensuring doc8, setuptools, and pbr are present
• make make spelling install libenchant-2-dev automatically on Linux when the enchant C library is missing
• extend the docs spelling wordlist for the formatter names used in the contribution guide

Test Plan

• local: make buildifier
• local: make addlicense
• local: make py-format
• local: make flake8
• local: make mypy
• local: make docstyle
• dev-0: make buildifier
• dev-0: make addlicense
• dev-0: make py-format
• dev-0: make flake8
• dev-0: make mypy
• dev-0: make docstyle
• dev-0: make doc-clean && make spelling
• dev-0: make spelling