fix: VPJWT JSON decoder in the issue-credential protocol

[yshyn-iohk]

Description:

• fix the JSON decoder in the VPJWT object of the issue-credential protocol

Alternatives Considered (optional):

Link to existing ADR (Architecture Decision Record), if any. If relevant, describe other approaches explored and the selected approach. Documenting why the methods were not selected will create a knowledge base for future reference, helping prevent others from revisiting less optimal ideas.

Checklist:

• My PR follows the contribution guidelines of this project
• My PR is free of third-party dependencies that don't comply with the Allowlist
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked the PR title to follow the conventional commit specification

[github-actions]

Integration Test Results

 23 files  ±0   23 suites  ±0   3s ⏱️ ±0s
 56 tests ±0   56 ✅ ±0  0 💤 ±0  0 ❌ ±0 
103 runs  ±0  103 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 3dbc7ee. ± Comparison against base commit fa1c04f.

[github-actions]

Unit Test Results

105 files  ±0  105 suites  ±0   20m 24s ⏱️ +12s
889 tests ±0  881 ✅ ±0  8 💤 ±0  0 ❌ ±0 
896 runs  ±0  888 ✅ ±0  8 💤 ±0  0 ❌ ±0 

Results for commit 3dbc7ee. ± Comparison against base commit fa1c04f.

[coveralls]

coverage: 48.657% (-0.07%) from 48.723%
when pulling 3dbc7ee on fix/vpjwt-jsondecoder
into fa1c04f on main.