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