feat: add eddsa mpcv2 custom signers in master express

[vibhavgo]

Summary

Add createEddsaMPCv2CustomSigners() to src/masterBitgoExpress/handlers/eddsa.ts, mirroring createEcdsaMPCv2CustomSigners in ecdsa.ts:

• Stateful round-generator closures capture the round 1 and round 2 AWM responses
• All three generators call the existing signMPCv2Round1/2/3() client methods with curveType: 'eddsa'
• Rounds 2/3 forward the encrypted data key, session state, and user GPG prv key captured from the previous round
• Explicit precondition errors for out-of-order round calls: Round 1 must be completed before Round 2, Round 2 must be completed before Round 3

Depends on the curveType client params from WCI-924 (base branch).

Testing

New src/__tests__/api/master/eddsaMpcv2Signing.test.ts (5 tests):

• Round 1 generator calls AWM with curveType: 'eddsa'
• Round 2 generator forwards round 1 encrypted state + curveType: 'eddsa'
• Round 3 generator forwards round 1 + round 2 encrypted state + curveType: 'eddsa'
• Round 2 before round 1 and round 3 before round 2 throw the exact error strings
  Regression: master suites (eddsaMpcv2Signing + ecdsa + eddsa) — 455 passing, 0 failing.

Ticket: WCI-925
Linear: https://linear.app/bitgo/issue/WCI-925

[linear-code]

WCI-925

[danielpeng1]

@claude review, focusing on gaps in types, casting, and other cleanups

[github-actions]

Claude encountered an error —— View job

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I'll analyze this and get back to you.

[Marzooqa]

Approving — reviewed against the ECDSA MPCv2 factory (createEcdsaMPCv2CustomSigners) and it mirrors it consistently. One non-blocking test-fixture note inline.

[Marzooqa]

🟡 Medium — Test fixtures can't prove captured-state forwarding. In eddsaMpcv2Signing.test.ts, the caller-supplied values are byte-identical to the captured-response values: encryptedRound1Session: 'mock-encrypted-round1-session' (fixture line 20 vs test input lines 100/134/172), encryptedUserGpgPrvKey (line 21 vs 99/133/139/171/185), encryptedRound2Session (line 26 vs 140/186). So assertions like line 112-113 pass even if the implementation forwarded the caller's param instead of the captured round-1 response — the PR's headline behavior isn't pinned. (encryptedDataKey is genuinely tested, since callers can't supply it.) Fix: use distinct strings, e.g. caller passes 'caller-round1-session' and assert the AWM param equals 'mock-encrypted-round1-session'.

Non-blocking — fine to address in a follow-up.