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

[Marzooqa]

• Add signTxRequest, signTxRequestForMessage, and signRequestBase to EddsaMPCv2Utils implementing WASM round 0 + API rounds 1 and 2
• Verify PGP signatures on BitGo round1Output and round2Output
• Return latestTxRequest after round 2 (round 3 + send deferred to next PR)
• Add signTxRequest unit tests using nock with real BitGo DSG sessions

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

TICKET: WCI-154

[linear-code]

WCI-154 [BitgoJS] Implement signRequestBase on EddsaMPCv2Utils - round 1 & round 2

[zahin-mohammad]

Reviewed against base #8687. Built locally — tsc --noEmit clean in sdk-core, signTxRequest.ts test passes (5/5).

The orchestration faithfully mirrors ECDSA's EcdsaMPCv2Utils.signRequestBase and the protocol structure looks right: WASM rounds 0/1/2 interleaved with API rounds 1/2/3, no client-side verification on round 3 (WP finalises). Nothing here freezes public API surface — the two new public methods (signTxRequest, signTxRequestForMessage) match the existing ITSSUtils/ECDSA shape, and signRequestBase is private. So everything below is follow-up safe.

Most significant finding is the missing mpcv2PartyId plumbing — backup signing is implicitly unsupported here even though the helpers from #8687 already accept the party id. ECDSA supports it; this is a behavioral gap vs the parallel path.

See inline comments.

[zahin-mohammad]

LGTM — gated on #8687 landing first (per the DO NOT MERGE label).

All 8 prior comments are resolved in this revision:

• ✅ Backup-signer plumbing: params.mpcv2PartyId now flows through DSG init + all three getSignatureShareRound{One,Two,Three} calls; signerShareType derived from partyId
• ✅ Typo handleIncomingMessagexs → handleIncomingMessages
• ✅ Assert ordering moved before Buffer.from(...) in both tx and message branches
• ✅ Defensive assert(userMsg2, ...) / assert(userMsg3, ...) after each handleIncomingMessages destructure
• ✅ Fragile [length - 1] share lookup replaced with new getBitgoSignatureShare(shares, signerShareType) helper in tss/common.ts that filters by from === BITGO && to === signerShareType
• ✅ messageRaw/bufferToSign footgun tracked as follow-up WCI-362
• ✅ Rate-limit retry tests added for rounds 1 and 3 (round 2 was already covered)
• ✅ Test coverage: BACKUP path ("signs a txRequest with backup key") + malformed round-3 response added. Round-2 tampered-signature is covered at the helper level in #8687 (verifyBitGoMessageRoundTwo should throw on a tampered message), so the orchestration-level gap is acceptable

Residual nits — non-blocking, address in a follow-up

1. Inconsistent error style in signRequestBase. The GPG-key check uses if (!bitgoGpgPubKey) throw new Error(...) while every other invariant in this method uses assert(...). Unifying on assert(bitgoGpgPubKey, 'Missing BitGo GPG key for MPCv2') would keep the failure-mode style consistent.
2. Unnamed boolean args to pickBitgoPubGpgKeyForSigning(true, params.reqId, txRequest.enterpriseId, true). This is a pre-existing pattern carried over from ECDSA, not introduced here, but the call site reads as flag soup. Worth a separate cleanup ticket against the helper signature (named-options object or named consts) so the pattern doesn't propagate further.
3. Round-2 orchestration-level negative test for a wrong-type response would mirror the existing should throw if round 1 response has wrong type and round the symmetry out. Not strictly needed since verifyBitGoMessageRoundTwo is unit-tested at the helper layer, but cheap to add.

None of these block — happy to approve as-is.

The base branch was changed.