feat(sdk-core): add EdDSA MPCv2 full 3-round signing orchestration

- Add signTxRequest, signTxRequestForMessage, and signRequestBase to
  EddsaMPCv2Utils implementing the full online DSG protocol:
  WASM round 0, API rounds 1-3, and sendTxRequest
- Verify PGP signatures on BitGo round1Output and round2Output
- Use pickBitgoPubGpgKeyForSigning with isEddsaMpcv2=true for ed25519 key
- Use decodeWithCodec for type-safe parsing of signature share responses
- Add comprehensive signTxRequest unit tests covering:
  - tx signing (all 3 rounds + send)
  - message signing (all 3 rounds + send)
  - 429 rate-limit retry handling (up to 3 retries)
  - rejection after 4+ consecutive 429 errors
  - rejection on wrong round1Output type

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TICKET: WCI-154

Author: Marzooqa