feat(jats): preserve external hyperlinks

[CreoEnMi-p]

Summary

Preserve external JATS hyperlinks from <ext-link xlink:href="..."> elements in the generated DoclingDocument.

This addresses the Hyperlinks subtask of #3583.

Changes

• propagate xlink:href through JATS inline parsing
• preserve hyperlinks together with nested inline formatting
• preserve hyperlink metadata on inline formula segments
• keep linked and unlinked text in separate inline runs
• prevent adjacent links with different targets from being merged
• support both absolute URLs and relative paths
• keep <ext-link> without xlink:href as normal text
• update the affected JATS groundtruth files

Internal <xref> references, citation parsing, and embedded images are intentionally left out of scope.

Tests

Added focused regression coverage for:

• linked text boundaries before and after <ext-link>
• nested formatting inside hyperlinks
• adjacent links with different targets
• missing xlink:href
• relative hyperlink targets
• inline formulas inside hyperlinks

Validation:

• uv run pytest tests/test_backend_jats.py -q — 40 passed
• Ruff format/check — passed
• ty check — passed
• Tach boundaries and module coverage — passed
• max-lines check — passed
• dprint — passed
• uv lock --locked — passed

On Windows, the full prek run --all-files cannot invoke the repository's python3 command because the local environment does not provide that executable alias. The affected Python-based hooks were run directly with the available Python executable and passed.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

[github-actions]

✅ DCO Check Passed

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