fix: preserve inline CSS background images

[itsafal]

Summary

Related to #1691. Crawl4AI currently omits image URLs declared in inline CSS background and background-image properties from its normal media and Markdown outputs.

This PR takes a deliberately narrow approach: it handles inline style attributes and routes the discovered URLs through the existing <img> extraction pipeline. It does not add computed-style extraction, external stylesheet parsing, new configuration, or new media models.

• Include extracted background images in media.images, cleaned_html, and Markdown.
• Preserve relative URL handling and existing image exclusion behavior.
• Skip data:, blob:, and javascript: URLs.
• Deduplicate repeated background URLs, including carousel clones.
• Use quote- and parenthesis-aware parsing for inline declarations.

This is intentionally smaller than the broader, previously closed CSS-background proposal in #1702, minimizing changes across the crawler pipeline while addressing the common inline-style case.

List of files changed and why

• crawl4ai/content_scraping_strategy.py - Extract inline CSS background URLs and route them through normal image processing.
• tests/test_css_background_images.py - Add regression coverage for extraction, Markdown, exclusions, URL handling, parser edge cases, and deduplication.

How Has This Been Tested?

• uv run --with pytest --with nest_asyncio --with tabulate pytest -q tests/test_css_background_images.py tests/test_issue_1484_css_selector.py tests/test_issue_1043_mermaid_svg.py (34 passed)
• git diff --check

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 (not applicable; this is an inline extraction behavior fix)
• 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