fix(markers): reject backslashes in marker values

[WSks-ui]

Fixes #1399.

Backslashes are currently accepted in quoted marker values, but they are interpreted as Python string escapes during parsing and emitted without escaping during serialization. As a result, a marker can change meaning after str(marker) and reparsing.

Reject unsupported backslashes while parsing marker values, following the marker grammar and the guidance in #1399. This prevents invalid input from silently changing semantics during serialization while preserving the existing output for valid markers.

Tests:

• Added marker parsing regressions for single and double quotes with one and two backslashes.
• Added a metadata serialization regression for Metadata.as_rfc822().
• The full test suite and property tests pass locally.
• Ruff, targeted mypy checks, build, and twine check --strict pass locally.