mbcollection: handle MB Unauthorized error

[snejus]

PR Summary

This change makes mbupdate fail gracefully when MusicBrainz returns 401 Unauthorized.

High-level impact

• Adds a MusicBrainz-specific HTTP error, UnauthorizedMBError, on top of the shared BeetsHTTPError request layer.
• Registers that error in MusicBrainzAPI, so authentication failures are translated into a clearer domain-specific exception.
• Updates beetsplug/mbcollection.py to catch BeetsHTTPError during collection sync and log a user-friendly error instead of crashing.
• Adds a regression test covering the unauthorized case and documents the behavior in docs/changelog.rst.

Architectural effect

• Keeps HTTP-to-domain error mapping centralized in beetsplug/_utils/musicbrainz.py and beetsplug/_utils/requests.py.
• Keeps mbcollection focused on sync flow and error handling, rather than raw HTTP details.
• Improves resilience of the mbcollection plugin without changing the normal successful update path.

Fixes #6651

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.48%. Comparing base (6dd6251) to head (96639ad).

Files with missing lines	Patch %	Lines
beetsplug/mbcollection.py	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6661      +/-   ##
==========================================
+ Coverage   72.46%   72.48%   +0.01%     
==========================================
  Files         161      161              
  Lines       20719    20731      +12     
  Branches     3280     3280              
==========================================
+ Hits        15014    15026      +12     
  Misses       4979     4979              
  Partials      726      726              

Files with missing lines	Coverage Δ
beetsplug/_utils/musicbrainz.py	97.35% <100.00%> (+0.05%)	⬆️
beetsplug/_utils/requests.py	97.75% <100.00%> (+0.02%)	⬆️
beetsplug/mbcollection.py	93.20% <92.30%> (+0.27%)	⬆️

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[Copilot]

Pull request overview

grug see PR make mbcollection not crash when MusicBrainz say 401 Unauthorized. grug like: plugin now catch nice beets HTTP error, log clear hint for user config, then keep program alive.

Changes:

• add MusicBrainz-specific UnauthorizedMBError and register it in MusicBrainzAPI HTTP error mapping
• catch BeetsHTTPError in mbcollection sync flow and log friendly error instead of raising up to importer
• add regression test and changelog note for 401 behavior

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
beetsplug/_utils/requests.py	allow BeetsHTTPError to accept custom message so domain errors can give better text
beetsplug/_utils/musicbrainz.py	define UnauthorizedMBError (401) and register in MusicBrainz API handler
beetsplug/mbcollection.py	catch BeetsHTTPError during collection update and log error instead of crash
test/plugins/test_mbcollection.py	add test that mbupdate logs unauthorized failure
docs/changelog.rst	document bugfix for mbcollection unauthorized case