Fix: Nut 05 mint response model

[callebtc]

Change response model according to cashubtc/nuts#136

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 8 lines in your changes missing coverage. Please review.

Project coverage is 64.28%. Comparing base (1766b6e) to head (ce6f1a2).
Report is 3 commits behind head on main.

❗ Current head ce6f1a2 differs from pull request most recent head d3568e5

Please upload reports for the commit d3568e5 to get more accurate results.

Files	Patch %	Lines
cashu/core/base.py	71.42%	2 Missing ⚠️
cashu/mint/ledger.py	71.42%	2 Missing ⚠️
cashu/mint/router.py	0.00%	2 Missing ⚠️
cashu/mint/router_deprecated.py	0.00%	1 Missing ⚠️
cashu/wallet/v1_api.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #564      +/-   ##
==========================================
- Coverage   65.02%   64.28%   -0.74%     
==========================================
  Files          91       91              
  Lines        8065     8080      +15     
==========================================
- Hits         5244     5194      -50     
- Misses       2821     2886      +65     

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