Fix line folding across escape sequences

[kingrubic]

Summary

• fixes [Bug] Description wrapping can create an invalid event #1501 by avoiding folded line breaks between escape prefixes and their escaped characters
• applies to TEXT backslash escapes (\n, \,, etc.) and RFC 6868 parameter escapes (^n, ^^, ^\')
• adds a regression test covering both escape families
• adds a towncrier bugfix fragment

Verification

• uv run pytest src/icalendar/tests/test_parsing.py -k '1501'
• uv run pytest src/icalendar/tests/test_parsing.py
• uv run pytest (16090 passed, 28 skipped)
• uv run ruff check src/icalendar/parser/string.py src/icalendar/tests/test_parsing.py
• uv run ruff format --check src/icalendar/parser/string.py src/icalendar/tests/test_parsing.py

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[read-the-docs-community]

Documentation build overview

📚 icalendar | 🛠️ Build #33279992 | 📁 Comparing dded291 against latest (937f6fc)

  🔍 Preview build  

13 files changed · ± 13 modified

± Modified

• 404.html
• genindex.html
• contribute/index.html
• reference/changelog.html
• contribute/documentation/build-check.html
• reference/api/icalendar.parser.content_line.html
• reference/api/icalendar.parser.html
• _modules/icalendar/cal/component.html
• _modules/icalendar/parser/content_line.html
• _modules/icalendar/parser/string.html
• and 3 more...

[angatha]

I commented #1501 and asked for a newer RFC version for content line folding. If no one could provide a version explicitly forbidding splitting thouse sequence, can you please include the reason (compatibility) why we aren't following the SHOULD from RFC 5545 or at least link the issue so everyone seeing that code knows that this is intentional.

Besides that, the code looks great.

I would wait for an answer on the issue before accepting that.