Expose structured access to JsonReply error code and supported_versions in FFI

[chavic]

JsonReply / WellKnownError are currently exposed as JSON strings only. Integrators must parse strings to discover the code and supported_versions.

Todo:

• Expose WellKnownError as a typed FFI record (code + supported_versions).
• Expose JsonReply with typed accessors rather than string parsing.
• Add binding tests for code matching (Unavailable, VersionUnsupported, etc.).
• Ensure error serialization still round‑trips across languages.

[spacebear21]

Closing per #1427 (comment)