fix(discogs): retry malformed search responses

[treyturner]

Summary

Retry Discogs searches once when the API returns an invalid JSON response. If the retry also fails, flow through existing error handling. Includes regression tests for successful recovery and retry exhaustion.

Reference: joalla/discogs_client#163

Originating stack trace:

discogs: Error searching Discogs: Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
  File "/usr/local/lib/python3.14/site-packages/beets/metadata_plugins.py", line 420, in _search_api
    response_data = self.get_search_response(params)
  File "/usr/local/lib/python3.14/site-packages/beetsplug/discogs/__init__.py", line 297, in get_search_response
    return [r.data for r in results.page(1)]
                            ~~~~~~~~~~~~^^^
  File "/usr/local/lib/python3.14/site-packages/discogs_client/models.py", line 347, in page
    data = self.client._get(self._url_for_page(index))
  File "/usr/local/lib/python3.14/site-packages/discogs_client/client.py", line 115, in _get
    return self._request('GET', url)
           ~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/local/lib/python3.14/site-packages/discogs_client/client.py", line 107, in _request
    body = json.loads(content)
  File "/usr/local/lib/python3.14/json/__init__.py", line 352, in loads
    return _default_decoder.decode(s)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "/usr/local/lib/python3.14/json/decoder.py", line 345, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.14/json/decoder.py", line 363, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Testing

• Full test suite on Python 3.14
• Ruff, mypy, and Sphinx lint

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.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.70%. Comparing base (74c2d98) to head (15359b3).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6912      +/-   ##
==========================================
+ Coverage   75.69%   75.70%   +0.01%     
==========================================
  Files         163      163              
  Lines       21412    21418       +6     
  Branches     3384     3384              
==========================================
+ Hits        16208    16215       +7     
+ Misses       4405     4403       -2     
- Partials      799      800       +1     

Files with missing lines	Coverage Δ
beetsplug/discogs/__init__.py	69.48% <100.00%> (+1.48%)	⬆️

... and 1 file with indirect coverage changes

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