fix(content-filter): honor preserve whitelist for excluded tags

[bong-u]

Summary

Fixes #2125

_remove_unwanted_tags decomposed by tag name, so preserve_tags and preserve_classes never reached elements whose tag is in excluded_tags.
It now iterates elements and skips the ones _is_preserved() matches, so both whitelists go through the guard that already existed.

This changes semantics for script and style, which are in excluded_tags too: preserve_tags=["script"] now really keeps them.
A user has to name the tag to get that, so I did not special-case it.

List of files changed and why

• crawl4ai/content_filter_strategy.py - filter by element instead of tag name in _remove_unwanted_tags so the whitelist is consulted; docstring updated to say the whitelist also overrides excluded_tags.
• tests/test_pruning_preserve_whitelist_1900.py - the old test_whitelist_does_not_override_excluded_tags asserted nothing and documented the bug as expected, so I inverted it, and added a case for a whitelisted class on an excluded tag.

How Has This Been Tested?

pytest tests/test_pruning_preserve_whitelist_1900.py - 21 passed.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes