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
- Support BACKUP party signing via mpcv2PartyId param
- 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
- Move getBitgoSignatureShare helper to tss/common.ts; use .find() by
  from/to instead of fragile array-index lookup
- Add defensive asserts after each WASM and API round
- Add comprehensive signTxRequest unit tests covering:
  - tx signing and message signing (all 3 rounds + send)
  - BACKUP party signing path
  - 429 rate-limit retry handling on rounds 1, 2, and 3
  - rejection after 4+ consecutive 429 errors
  - rejection on wrong round1Output type
  - rejection on malformed round3 response

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

TICKET: WCI-154

Author: Marzooqa