fix(core): use cls instead of self in PostMeltResponse.from_melt_quote

[KvngMikey]

Fixes #1006

Summary

PostMeltQuoteResponse.from_melt_quote was decorated with @classmethod but had two bugs:

• The first parameter was named self instead of cls, inconsistent with Python classmethod conventions.
• The return statement hardcoded PostMeltQuoteResponse.model_validate(to_dict) instead of cls.model_validate(to_dict), silently returning a PostMeltQuoteResponse instance even when called on a subclass.

Changes

• Renamed first parameter from self to cls
• Replaced PostMeltQuoteResponse.model_validate(to_dict) with cls.model_validate(to_dict)

This now matches the existing pattern in PostMintQuoteResponse.from_mint_quote in cashu/core/models/mint_quote.py.

[Copilot]

Pull request overview

This PR fixes PostMeltQuoteResponse.from_melt_quote so the classmethod uses cls, preserving subclass-friendly construction consistent with other model factory methods.

Changes:

• Renames the classmethod receiver from self to cls.
• Uses cls.model_validate(...) instead of hard-coding PostMeltQuoteResponse.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.35%. Comparing base (2cf3598) to head (3908d33).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1007      +/-   ##
==========================================
- Coverage   75.44%   75.35%   -0.10%     
==========================================
  Files         110      110              
  Lines       11798    11798              
==========================================
- Hits         8901     8890      -11     
- Misses       2897     2908      +11     

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