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

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.61%. Comparing base (73605d5) to head (c1de81d).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/ftintitle.py	92.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6716   +/-   ##
=======================================
  Coverage   74.60%   74.61%           
=======================================
  Files         163      163           
  Lines       20830    20852   +22     
  Branches     3283     3289    +6     
=======================================
+ Hits        15541    15559   +18     
- Misses       4538     4540    +2     
- Partials      751      753    +2     

Files with missing lines	Coverage Δ
beets/plugins.py	88.97% <100.00%> (+0.13%)	⬆️
beetsplug/ftintitle.py	89.43% <92.30%> (-1.63%)	⬇️

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