fix(sdjwt): dump JsonWebKey in json mode so enums are serialized as strings

[aksharprabhudesai]

Description

In ParsedToken.cnf_jwk, jwk_model.model_dump() without mode="json" leaves enum fields like use as Python enum instances (Use.sig) rather than strings. Passing those into jwcrypto.jwk.JWK causes signature verification on delegation hops to fail with InvalidJWSSignature.

This sets mode="json" on the model_dump() call so enum values are serialized as strings, and adds test coverage for cnf_jwk and delegation hops with use="sig".

• Follow the CONTRIBUTING Guide.
• Make your Pull Request title in the https://www.conventionalcommits.org/ specification.
• Ensure the tests and linter pass
• Appropriate docs were updated (if necessary)

Fixes #372 🦕