Fixes #3993.

mkdocs serve reloads the configuration for rebuilds, but hook modules were cached by Hooks._load_hook(). If a watched hook file changed, the subsequent config validation reused the old module object instead of executing the updated hook file.

This removes the cross-validation cache for hook modules and executes the current hook source on each config load, while preserving the existing module metadata, sys.modules registration, and temporary hook-directory sys.path handling. The regression test also covers a same-size/same-mtime rewrite so reloads are not dependent on Python bytecode-cache freshness.

Tests run:

• python -m unittest mkdocs.tests.config.config_options_tests.HooksTest.test_hooks_reload_when_file_changes
• python -m unittest mkdocs.tests.config.config_options_tests.HooksTest
• python -m unittest mkdocs.tests.config.config_options_tests
• python -m unittest mkdocs.tests.config.config_options_legacy_tests.HooksTest
• python -m unittest discover -s mkdocs\tests\config -p "*_tests.py"
• ruff check mkdocs\config\config_options.py mkdocs\tests\config\config_options_tests.py
• python -X utf8 -m isort --check-only --diff mkdocs\config\config_options.py mkdocs\tests\config\config_options_tests.py
• python -X utf8 -m black --workers 1 -q --check --diff --line-ranges 1-8 --line-ranges 1186-1208 mkdocs\config\config_options.py
• python -X utf8 -m black --workers 1 -q --check --diff --line-ranges 2374-2406 mkdocs\tests\config\config_options_tests.py
• git diff --check