fix(modify): parse typed values when selecting objects at the confirmation prompt

[cycsmail]

Description

Fixes #4880.

When you modify a non-string field (e.g. added) and choose select at the "Really modify" prompt, beets crashed. The select callback re-ran print_and_modify with the raw assignment strings (mods) instead of the per-object parsed values, so a date string was handed straight to a field expecting a timestamp.

Pulled the parsing into a small parse_mods(obj) helper and used it in both the main modify loop and the select callback, so values get parsed in both paths.

Added a regression test that modifies added via the select path and checks the stored timestamp.

To Do

• Documentation (bugfix, no doc change needed)
• Changelog
• Tests

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reuse parsed modifications for select prompts

When the user chooses select after a templated modification such as beet modify 'title=${title} - append', changed contains the same objects that the preview loop already mutated. Calling parse_mods(o) here evaluates the template against that mutated value, so confirming an individual object applies the template a second time and stores Tracktitle - append - append instead of the single append shown by the initial preview/all path. The select callback should reuse the per-object values computed before the preview or evaluate against a fresh/original object.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.93%. Comparing base (e700135) to head (22059fd).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6782   +/-   ##
=======================================
  Coverage   74.92%   74.93%           
=======================================
  Files         163      163           
  Lines       21148    21150    +2     
  Branches     3337     3337           
=======================================
+ Hits        15846    15848    +2     
  Misses       4539     4539           
  Partials      763      763           

Files with missing lines	Coverage Δ
beets/ui/commands/modify.py	95.87% <100.00%> (+0.08%)	⬆️

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