fix: Respect <base> tag for relative link resolution in html2text (#1680)

[YuriNachos]

Summary

• Added tag handling in HTML2Text and CustomHTML2Text
• Updates baseurl when <base href="..."> is encountered
• Fixes relative link resolution to follow HTML standards

Fixes

Fixes #1680

Details

The HTML2Text class was ignoring the <base> tag, causing relative links
to be resolved against the initial page URL instead of the base URL
specified in the HTML document.

According to HTML standards, the <base> element specifies the base URL
for all relative URLs in a document. This is commonly used in:

• Content Management Systems (CMS)
• Single Page Applications (SPA)
• Documentation sites
• Sites with complex URL structures

Changes

1. HTML2Text.handle_tag(): Added <base> tag detection and baseurl update
2. CustomHTML2Text.handle_tag(): Added same handling before preserved tags processing

Example

Before:

• Page URL: https://example.com/index.php/page.html
• Base tag: <base href="https://example.com/">
• Relative link: files/document.pdf
• Resolved to: https://example.com/index.php/files/document.pdf ❌

After:

• Resolved to: https://example.com/files/document.pdf ✅

Test plan

• Code review confirms tag is properly handled
• Both HTML2Text and CustomHTML2Text updated
• baseurl is updated when tag is encountered
• Follows HTML standard behavior

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