feat(core): Updates to nut18 and nut26 payment request

[KvngMikey]

implements cashubtc/nuts#381 to keep Nutshell up to date !

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.36%. Comparing base (e665f46) to head (11612fe).
⚠️ Report is 27 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1059      +/-   ##
==========================================
+ Coverage   74.64%   75.36%   +0.72%     
==========================================
  Files         111      111              
  Lines       12387    12542     +155     
==========================================
+ Hits         9246     9452     +206     
+ Misses       3141     3090      -51     

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[a1denvalu3]

@KvngMikey If I am not mistaken this only adds support for decoding such payment requests with the additional fields but does not add any logic to handle them or to create a payment request with them.

[KvngMikey]

@KvngMikey If I am not mistaken this only adds support for decoding such payment requests with the additional fields but does not add any logic to handle them or to create a payment request with them.

@a1denvalu3 you're right that this PR only adds encoding/decoding for the new ms / fr / sm fields and doesn't add enforcement logic.

Happy to either extend this PR with that handling + a request-creation path, or land this as the serialization layer and do the behavior in a focused follow-up. Which would you prefer?

[a1denvalu3]

Happy to either extend this PR with that handling + a request-creation path

Yes please

[robwoodgate]

The nut has been updated: ms (mint_strict) is now flipped to mp (mint_preferred) to avoid the omitted=true antipattern.