Fix JWT Serialization

[spruceduncan]

Summary

Fixes double-encoding of JWT credential payloads for jwt_vc_json and jwt_vc_json-ld formats in RawCredential::from_oid4vci.

The StandardFormat::W3c arm was calling serde_json::to_vec(&credential.value) to produce the payload bytes. When the credential value is a serde_json::Value::String (which it is for jwt_vc_json and jwt_vc_json-ld), this JSON-encodes the string again, wrapping the raw JWT bytes in literal " characters. This causes a FormatException: Invalid base64 data error on the client side when attempting to decode the JWT.

The other string-based format arms (DcSdJwt, MsoMdoc, vc+sd-jwt) already handle this correctly by extracting the inner string with pattern matching before calling .into_bytes(). This PR applies the same pattern to the W3c arm, while preserving the serde_json::to_vec path for ldp_vc where the credential value is a JSON object.

Bug introduced in ecf7eb2 ("Migrate OID4VCI 1.0 (#269)").

Changes

• rust/src/oid4vci/credential.rs: Extract string values before converting to bytes in the W3c arm, matching the pattern used by all other string-based formats
• rust/src/tests.rs: Add regression tests for jwt_vc_json, jwt_vc_json-ld, and ldp_vc payload encoding