fix(invoice): correctly parse assignment_name in RgbInvoice::from_str

[astral-bitlight]

Description

This PR addresses a (seems like?) bug in the parsing logic for RgbInvoice strings where the optional assignment_name field was not correctly handled during deserialization, leading to data loss during round-trip serialization/deserialization.

Updates

The impl FromStr for RgbInvoice has been updated to correctly handle both possible path structures:

1. contract/schema/assignment_data (where assignment_name is None)
2. contract/schema/assignment_name/assignment_data (where assignment_name is Some(...))

Testing

• A new test case, parse_with_assignment_name, has been added to specifically test the round-trip serialization and deserialization of an RgbInvoice containing an assignment_name. (This test now passes, confirming the fix. )
• All existing tests continue to pass.