feat(jats): embed local figure images

[teachershuang]

Summary

• preserve local images referenced by direct <fig><graphic xlink:href="..."> elements in file-based JATS documents
• attach decoded images as ImageRef values on the generated PictureItem while preserving captions
• keep stream inputs and unavailable, remote, or unsafe references as placeholders, and confine local paths to the JATS document directory

Part of #3583.

Testing

• conda run -n docling-contrib python -m pytest -q tests/test_backend_jats.py (42 passed)
• focused image-resource tests with pytest-cov (8 passed; every executable line added in jats_backend.py covered)
• conda run -n docling-contrib python -m pytest -q tests/test_backend_optional_dependencies.py tests/test_input_doc.py (29 passed)
• changed-file prek hooks: trailing whitespace, EOF, merge conflict, large files, Ruff lint/format, ty, Tach, module coverage, and 1500-line limit
• repository read-only checks: Ruff format/lint, ty (warning-mode exit 0), Tach, module coverage, dprint, and locked dependency resolution
• the standalone default 1000-line max-lines check reports existing over-limit files on current origin/main; the changed-file 1500-line hook passes

Scope

Remote URL fetching, inline graphics, and nested <alternatives> graphics remain out of scope for this focused change.

Checklist:

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

[github-actions]

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[mergify]

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• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/backend/xml/jats_backend.py	88.88%	3 Missing ⚠️

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