feat(nut04/05): optional amount

[asmogo]

Description

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Notes to the reviewers

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Suggested CHANGELOG Updates

CHANGED

ADDED

REMOVED

FIXED

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Checklist

• I followed the code style guidelines
• I ran just quick-check before committing
• If the Wallet API was modified (added/removed/changed), I have reflected those changes in the FFI bindings (crates/cdk-ffi)

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.59%. Comparing base (d481edc) to head (fdf427a).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/cdk-payment-processor/src/proto/server.rs	0.00%	2 Missing ⚠️
crates/cdk-fake-wallet/src/lib.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2146      +/-   ##
==========================================
+ Coverage   71.55%   71.59%   +0.04%     
==========================================
  Files         356      356              
  Lines       73999    74131     +132     
==========================================
+ Hits        52950    53077     +127     
- Misses      21049    21054       +5     

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