fix(doclang): sanitize XML-illegal text characters

[AkhilTrivediX]

Summary

Preserves XML-illegal text characters as visible Unicode markers during DocLang serialization, preventing parseString from raising on content such as PPTX soft line breaks (\x0b).

Fixes #686

Changes

• Normalize XML 1.0-illegal characters in the shared DocLang text-escaping path.
• Represent each removed control character as [U+XXXX] so the source anomaly remains visible.
• Add regression coverage for compact and pretty-printed output.

Type of change

• Bug fix

How Has This Been Tested?

• uv run pytest test/test_serialization_doclang.py -k xml_illegal -q
• uv run mypy docling_core/transforms/serializer/doclang.py test/test_serialization_doclang.py
• uv run ruff check docling_core/transforms/serializer/doclang.py test/test_serialization_doclang.py
• uv run ruff format --check docling_core/transforms/serializer/doclang.py test/test_serialization_doclang.py

Checklist

• I added regression coverage for the reported failure.
• The focused checks above pass locally.

[github-actions]

✅ DCO Check Passed

Thanks @AkhilTrivediX, all your commits are properly signed off. 🎉

[mergify]

Merge Protections

🔴 1 of 2 protections blocking · waiting on 👀 reviews

	Protection	Waiting on
🔴	Require two reviewer for test updates	👀 reviews
🟢	Enforce conventional commit	—

🔴 Require two reviewer for test updates

Waiting for

• #approved-reviews-by >= 2

This rule is failing.

When test data is updated, we require two reviewers

• #approved-reviews-by >= 2

---

Show 1 satisfied protection

🟢 Enforce conventional commit

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!