fix(ftintitle): parenthetical features

[treyturner]

Description

Fixes #6715: ftintitle handling for featured artists wrapped in parentheses or square brackets.

Before this change, parenthetical features were split at the feature token but the surrounding delimiters were left behind on both sides. For example:

Alice (ft. Bob) - Song

became:

Alice ( - Song ft. Bob)

With this PR, we get the intended:

Alice - Song ft. Bob

The same cleanup applies to square brackets, e.g. Alice [ft. Bob].

Details

• Strip a matching () or [] pair when it encloses the feature marker split.
• Ensures artist names with parens or brackets retain them, e.g. Alice ft. (Bob) still extracts (Bob).
• Handle album-artist matching when the album artist is the bracketed featured artist, e.g. Alice [ft. Bob] on Bob’s album extracts Alice.
• Doc update explaining the behavior.

Tests

• Added focused ftintitle cases for:
  • functional parenthetical feature cleanup
  • bracketed find_feat_part album-artist behavior
  • split_on_feat delimiter cleanup and non-cleanup boundaries

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.53%. Comparing base (beff630) to head (0806299).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6716      +/-   ##
==========================================
- Coverage   74.54%   74.53%   -0.01%     
==========================================
  Files         162      162              
  Lines       20832    20842      +10     
  Branches     3298     3300       +2     
==========================================
+ Hits        15529    15535       +6     
- Misses       4548     4550       +2     
- Partials      755      757       +2     

Files with missing lines	Coverage Δ
beetsplug/ftintitle.py	90.22% <100.00%> (-0.84%)	⬇️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.