Fixes #4045.

What changed

• Keep setColorMode() updating the Bootstrap data-bs-theme attribute even when theme.highlightjs is disabled.
• Make highlight.js stylesheet toggling conditional on the hljs-light/hljs-dark link elements actually existing.
• Add a focused JavaScript smoke regression for the missing-highlightjs-links case while preserving stylesheet toggling when highlight.js is enabled.

Verification

• C:\hades\oss\.venvs\mkdocs-4045\Scripts\python.exe -m unittest mkdocs.tests.theme_tests.ThemeTests.test_mkdocs_theme_darkmode_without_highlightjs_links
• C:\hades\oss\.venvs\mkdocs-4045\Scripts\python.exe -m unittest mkdocs.tests.theme_tests
• C:\hades\oss\.venvs\mkdocs-4045\Scripts\python.exe -m ruff check mkdocs\tests\theme_tests.py
• C:\hades\oss\.venvs\mkdocs-4045\Scripts\python.exe -m black --target-version py313 --check mkdocs\tests\theme_tests.py
• C:\hades\oss\.venvs\mkdocs-4045\Scripts\python.exe -m isort --check-only --diff mkdocs\tests\theme_tests.py
• npx --yes jshint mkdocs\themes\mkdocs\js\darkmode.js
• mkdocs build --strict smoke with theme.highlightjs: false, color_mode: auto, and user_color_mode_toggle: true
• git diff --check