feat: add preferred_mints to payment requests

[a1denvalu3]

Summary

• Add support for preferred_mints to Cashu Payment Requests (NUT-18 and NUT-26)
• Update PaymentRequest and CreateRequestParams structs with preferred_mints across core, wallet, CLI, and FFI
• Ensure mints and preferred_mints are mutually exclusive during creation and decoding
• Update wallet pay_request selection logic to prioritize preferred_mints over fallback mints if they have sufficient balance

[codecov]

Codecov Report

❌ Patch coverage is 79.53488% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.25%. Comparing base (bdf9dc6) to head (8eae376).

Files with missing lines	Patch %	Lines
crates/cdk/src/wallet/payment_request.rs	53.44%	27 Missing ⚠️
crates/cashu/src/nuts/nut18/payment_request.rs	86.56%	9 Missing ⚠️
crates/cdk-ffi/src/types/payment_request.rs	78.12%	7 Missing ⚠️
crates/cashu/src/nuts/nut26/error.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1925      +/-   ##
==========================================
+ Coverage   65.14%   65.25%   +0.10%     
==========================================
  Files         329      329              
  Lines       56742    56939     +197     
==========================================
+ Hits        36965    37155     +190     
- Misses      19777    19784       +7     

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[lescuer97]

LGTM. The errors in the CLI seem to be clippy and other errors not related to the code

[thesimplekid]

Can you rebase instead of merge https://github.com/cashubtc/cdk/blob/main/DEVELOPMENT.md#keeping-your-branch-up-to-date please

[a1denvalu3]

Can you rebase instead of merge https://github.com/cashubtc/cdk/blob/main/DEVELOPMENT.md#keeping-your-branch-up-to-date please

I usually rebase, but the update button under the PR was too tempting this time.