feat(mint): align NUT-04 with amount_paid/issued + updated_at

[a1denvalu3]

This PR aligns the nutshell codebase with the Cashu specification change in cashubtc/nuts PR #377 (Align NUT-04 mint quote accounting with amount_paid / amount_issued).

Changes:

• Added amount_paid, amount_issued, and updated_at properties to MintQuote (with amount_paid remaining 0 when state is PENDING per requirements).
• Added amount_paid, amount_issued, and updated_at fields to PostMintQuoteResponse (making them Optional to maintain backward compatibility with older mints).
• Updated wallet-side parsing in from_resp_wallet to prioritize using amount_paid/amount_issued over state when present, handling monotonic checks (updated_at and non-decreasing amounts) for stale responses.
• Updated API router on the mint side to return the new fields.
• Added unit test coverage for the new properties and serialization values.

[codecov]

Codecov Report

❌ Patch coverage is 81.15942% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.32%. Comparing base (13ba163) to head (e1ec396).
⚠️ Report is 8 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/core/base.py	77.58%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1044      +/-   ##
==========================================
+ Coverage   74.79%   75.32%   +0.52%     
==========================================
  Files         111      111              
  Lines       12413    12496      +83     
==========================================
+ Hits         9284     9412     +128     
+ Misses       3129     3084      -45     

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