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 74.84%. Comparing base (bf879d5) to head (417fbe5).
⚠️ Report is 7 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6782      +/-   ##
==========================================
- Coverage   74.84%   74.84%   -0.01%     
==========================================
  Files         163      163              
  Lines       20949    20948       -1     
  Branches     3300     3299       -1     
==========================================
- Hits        15680    15679       -1     
  Misses       4511     4511              
  Partials      758      758              

Files with missing lines	Coverage Δ
beets/ui/commands/modify.py	94.11% <100.00%> (+0.08%)	⬆️

... and 2 files with indirect coverage changes

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