fix(mint): fix rotate_next_keyset

[vicjuma]

Summary

cashu/mint/keysets.py:37 (rotate_next_keyset) failed to update selected_keyset_counter inside its selection loop, so the "select the active keyset with the highest counter" comparison always ran against its initial value instead of the running maximum. With a single active keyset per unit — the only case exercised so far — this has no observable effect, since there's nothing to compare against. However, the moment more than one active keyset exists for the same unit (a state NUT-02 permits), selection becomes dependent on dict iteration order rather than actual counter value, which can silently select the wrong keyset for rotation.

Fixing this is low-risk and preserves current single-keyset behavior exactly, while making the function correct if that assumption ever doesn't hold — whether from future multi-keyset support, migrations, or manual DB state.

Also replaced a stray keyset.id reference in the final debug log with selected_keyset.id. keyset was a leftover loop variable from the selection loop above and no longer holds a meaningful value once the loop exits; using it there could log the wrong keyset ID as the one deactivated.

Changes

• rotate_next_keyset: correctly track selected_keyset_counter on each iteration
• rotate_next_keyset: fix debug log to reference selected_keyset.id instead of stale keyset.id

Testing

pytest tests/mint/test_mint_keysets.py::test_keyset_rotation

No behavioral change for the existing single-active-keyset code path, so the existing test suite passes unmodified.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.60%. Comparing base (1853902) to head (bb33a82).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/mint/keysets.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1097      +/-   ##
==========================================
- Coverage   74.63%   74.60%   -0.03%     
==========================================
  Files         115      115              
  Lines       13200    13201       +1     
==========================================
- Hits         9852     9849       -3     
- Misses       3348     3352       +4     

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[a1denvalu3]

Already being fixed in #1058