Prompt format string for literal backslash

[rolandwalker]

Description

In case the user wanted to write a literal \h for some reason, that was previously not possible.

This still doesn't work right for \x, since that is interpreted later, something we can address by unifying.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Correctness (edge-case regression): mycli/main.py:1705 introduces a fixed sentinel ('\ufffc_backslash') and later blindly converts it back to \ at :1726.
   If a user’s prompt literally contains that substring, it will be mutated unexpectedly.
   Action: use a collision-safe strategy (e.g., generate a per-call unique sentinel not present in the input, or parse escapes without placeholder substitution).

2. Missing test coverage: this PR changes prompt parsing logic but adds only config comments (test/myclirc:125); no executable tests verify the new behavior.
   Action: add tests in test/test_main.py for:

• escaped token stays literal ("\\\\h" -> "\h")
• plain literal backslash ("\\\\" -> "\\")
• mixed escaped + real tokens (e.g., "\\\\h \\h")

No security concerns found in the introduced changes.