fix(markdown): resolve relative HTML base URLs

[ktz03]

Summary

Relative <base href> values discard the document origin during Markdown generation, producing incorrect link, image, and citation URLs. Resolve the base element against the document URL once; for example, ../assets/ on https://example.test/pages/index.html now resolves guide.html to https://example.test/assets/guide.html.

List of files changed and why

• crawl4ai/html2text/__init__.py: resolve relative base URLs in HTML2Text and let CustomHTML2Text delegate without overwriting or resolving the base twice.
• tests/unit/test_markdown_base_url.py: exercise both converters and the public Markdown generator, with relative bases and absolute, absent, and empty base controls.

How Has This Been Tested?

On Windows with Python 3.12.14:

python -m pytest tests/unit/test_markdown_base_url.py tests/test_table_gfm_compliance.py tests/test_table_rowspan_grid.py tests/test_markdown_generator_validation_1880.py tests/general/test_async_markdown_generator.py tests/general/test_content_source_parameter.py -q
67 passed

The new regression suite has 16 failures and 12 passing controls on the unchanged develop base; all 28 cases pass with the fix. It covers path-relative, parent-relative, root-relative, and protocol-relative base URLs, with citations enabled and disabled. An independent public-API check also passes 96 scenarios across document URLs, base URLs, link forms, and both citation modes. Black checks pass for the new test file and git diff --check passes. A scoped source Black check reports four existing whitespace-only lines on both unchanged develop and the fixed code. Browser, live crawling, and the full repository test suite were not run.

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

No documentation change is needed for correcting the existing base URL behavior. The selected local suites pass as detailed above; the full suite was not run.