fix: remove state from MintQuoteCustomResponse and update default extra

[asmogo]

… quote

• Add #[serde(default)] to MintQuoteCustomResponse::state so mints that omit the field (returning amount_paid/amount_issued instead) deserialize correctly, defaulting to QuoteState::Unpaid instead of failing
• Replace extra.unwrap_or_default() (empty string) with extra.as_deref().unwrap_or("{}") in wallet/issue and cdk-payment-processor so serde_json::from_str never receives an empty string, which caused an EOF parse error when extra was None

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 81.14035% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.26%. Comparing base (e06f337) to head (798acd2).

Files with missing lines	Patch %	Lines
crates/cdk-common/src/mint.rs	13.33%	13 Missing ⚠️
crates/cdk/src/wallet/issue/mod.rs	50.00%	13 Missing ⚠️
crates/cdk-common/src/wallet/mod.rs	0.00%	6 Missing ⚠️
crates/cashu/src/nuts/nut04.rs	81.81%	4 Missing ⚠️
crates/cdk-ffi/src/types/quote.rs	0.00%	3 Missing ⚠️
crates/cdk-common/src/mint_quote.rs	98.18%	1 Missing ⚠️
crates/cdk-payment-processor/src/proto/mod.rs	0.00%	1 Missing ⚠️
...rates/cdk/src/wallet/mint_connector/http_client.rs	98.41%	1 Missing ⚠️
crates/cdk/src/wallet/test_utils.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1915      +/-   ##
==========================================
+ Coverage   65.16%   65.26%   +0.09%     
==========================================
  Files         330      330              
  Lines       56742    56923     +181     
==========================================
+ Hits        36976    37148     +172     
- Misses      19766    19775       +9     

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[thesimplekid]

@asmogo I just squashed and rebased this. Changes LGTM will just let ci finish.