Add test coverage for string_utils.py

[rolandwalker]

Description

Add test coverage for string_utils.py, and fix up the changelog.

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]

Findings

1. Missing security-relevant escape-sequence coverage (low)
   File: test/pytests/test_string_utils.py
   Current tests only validate SGR-style ANSI (\x1b[31m...). Terminal-title sanitization should also be locked down for OSC/control-sequence forms (for example \x1b]0;...\x07), since those are common in title-setting/injection paths.
   Action: add a test case for OSC-style sequences to ensure sanitize_terminal_title() strips them.

2. Missing carriage-return edge case (low)
   File: test/pytests/test_string_utils.py
   \n and \r\n are tested, but bare \r is not. Current implementation removes \r (via control-char regex) rather than spacing it, which can collapse words unexpectedly.
   Action: add a test for bare \r and assert intended behavior ("a b" vs "ab"), so this is explicit.

No correctness or regression bug found in the new test code itself.

Testing note

I could not execute the test locally because uv is not installed in this environment (uv: command not found).