fix: respect <base> tag in html2text conversion

[biplavbarua]

Description

The html2text parser currently ignores the HTML <base> tag. This PR adds logic to detect the <base> tag and update the parser's base URL accordingly, ensuring that relative links are resolved correctly.

Related Issue

Fixes #1680

Verification

• Added a local unit test tests/test_base_tag_local.py which passes.
• Confirmed that links are resolved against the href specified in the <base> tag.

[biplavbarua]

LGTM! This fixes the critical issue of broken relative links when a tag is present.
Minor implementation note: Since html2text is a stream parser, this update applies to all subsequent tags. While the HTML spec mandates the first tag in controls the whole document (including elements before it), strictly parsing that requires a two-pass approach or a full DOM tree.
Given the constraints of HTMLParser, this is the correct pragmatic solution.
Verified that urljoin logic correctly handles the accumulation/replacement of the base path.

[biplavbarua]

Re-verified local build against latest master. Fix is stable.