Expose typed error variants for receiver errors in FFI bindings

[chavic]

Receiver‑side errors like PayloadError, RequestError, and SessionError are not exported as typed FFI enums. Callers can’t branch on error causes, which blocks recovery logic in clients and reduces testability.

Todo:

• Export PayloadError, RequestError, SessionError to FFI.
• Preserve variant details (e.g., PSBT parse vs fee mismatch vs prev‑txout mismatch).
• Add unit tests that assert specific error variants in JS/Python/Dart.
• Update integration tests to assert typed errors on bad inputs/headers.
• Document expected error mappings in FFI comments.

[spacebear21]

I don't think this is an issue, these internal variants are explicitly opaque. see comment here #1428 (review) and some relevant discussion here #403