fix: persist melt extra_json

[thesimplekid]

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 98.02372% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.12%. Comparing base (2fed066) to head (5b333b0).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/cdk/src/mint/subscription.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1910      +/-   ##
==========================================
+ Coverage   63.76%   64.12%   +0.36%     
==========================================
  Files         329      329              
  Lines       55178    55407     +229     
==========================================
+ Hits        35184    35531     +347     
+ Misses      19994    19876     -118     

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