Test coverage: expand invoice-token SEP-41 behavior

[Josue19-08]

Description

Expanded invoice-token SEP-41 coverage with additional success and failure cases for transfer, approve, transfer_from, mint, burn, and burn_from.

Closes #6

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• 🎨 UI/UX improvement
• ♻️ Code refactoring
• ✅ Test addition or update
• 🔧 Configuration change

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Testing

How to Test

1. Run cargo test -p invoice-token.
2. Run cargo test --workspace.
3. Run cargo clippy --workspace --all-targets -- -D warnings.

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• E2E tests added/updated (if applicable)
• Manual testing completed

Additional Notes

No production contract behavior changed in this PR; this issue is test coverage only.

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