fix: single-quote YAML strings with backslashes for safe round-tripping

[NIK-TIGER-BILL]

Related Issues

• fixes Invalid escape sequences in regex in the pipeline YAML #11093

Proposed Changes:

When serializing pipelines containing regex patterns (e.g. `\b\w+\b`) or file paths with backslashes, PyYAML emits plain scalars that can be misinterpreted as YAML escape sequences on load. On Python 3.13+ this produces `SyntaxWarning`, and on some configurations it causes `ReaderError: unacceptable character #x0008`.

Fix: Override `YamlDumper.represent_str` to emit single-quoted YAML scalars for any string containing a backslash. In single-quoted YAML scalars, no escape sequences are interpreted, so the round-trip is always safe.

Before:

After:

Strings without backslashes continue to be emitted as plain scalars (no change).

How did you test it?

• Added 6 new unit tests in `test/marshal/test_yaml.py`:
  • Single backslash sequence round-trip
  • Complex regex round-trip
  • Windows path round-trip
  • No-backslash strings unchanged
  • Single-quote style verification
  • Full Pipeline round-trip with DocumentCleaner regex parameters
• All 9 tests pass (3 existing + 6 new)

Notes for the reviewer

This is a minimal, targeted fix. An alternative approach would be to always quote all strings, but that would make the YAML output significantly more verbose for no additional safety benefit.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I have used conventional commit type `fix:` for my PR title.
• I have documented my code.
• I have added a release note file.
• I have run pre-commit hooks and fixed any issue.

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[sjrl]

@anakin87 reassigning to you for review since you're assigned the issue

[anakin87]

@NIK-TIGER-BILL please sign the CLA, then I'll review

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