typing: type and refactor duplicates plugin

[snejus]

Part of #6924.

What changed

• Types added to beetsplug/duplicates.py.
• Refactors beetsplug/duplicates.py so album and item commands now share one execution path through _run_command().
• Replaces the old generic _merge() dispatch with an explicit merge_func passed into _duplicates(), so album and item merge behavior stays separate but simpler.
• Tightens typing across the plugin by using clearer Album, Item, AlbumOrItem, and LibModel annotations in the main helper methods.

Why it matters

• The plugin's architecture is now more direct: command setup decides 'albums' vs 'items', and the shared duplicate-processing flow handles the rest.
• This removes branching and type ambiguity from the core duplicate logic, which makes the code easier to follow and safer to change.
• Functional behavior stays the same at a high level, but the plugin is now easier to maintain, reason about, and extend.

[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.

[Copilot]

Pull request overview

grug see PR make beetsplug/duplicates.py more typed, and refactor so album + item modes share one main flow. goal look like: fewer branches, clearer types, still same user behavior.

Changes:

• add typing across duplicates plugin, incl. AlbumOrItem + clearer helper signatures
• refactor command path so both album + item go through new _run_command()
• replace old generic merge dispatch with explicit merge_func passed into _duplicates()

Suppressed comments (1)

beetsplug/duplicates.py:287

• grug see log line still say "on item" but var now model (and can be album). log text should say "model" so debug not lie.

            self._log.debug(
                "key {} on item {.filepath} cached:not computing checksum",
                key,
                model,
            )

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[codecov]

Codecov Report

❌ Patch coverage is 57.81250% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.07%. Comparing base (aa2190d) to head (1b58420).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/duplicates.py	57.81%	20 Missing and 7 partials ⚠️

Additional details and impacted files

@@                         Coverage Diff                          @@
##           type-mpdstats-thumbnails-plugins    #6947      +/-   ##
====================================================================
+ Coverage                             76.02%   76.07%   +0.04%     
====================================================================
  Files                                   164      164              
  Lines                                 21615    21599      -16     
  Branches                               3342     3338       -4     
====================================================================
- Hits                                  16433    16431       -2     
+ Misses                                 4386     4374      -12     
+ Partials                                796      794       -2     

Files with missing lines	Coverage Δ
beetsplug/duplicates.py	52.28% <57.81%> (+3.18%)	⬆️

... and 1 file with indirect coverage changes

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