feat(sdk-core): add externalSigner createOfflineRound2Share handler

[vibhavgo]

Add the EdDSA MPCv2 offline round-2 handler for external signer flows. It restores the encrypted round-1 DSG session and produces the round-2 share plus encrypted carry-over state for round 3.

• Restore user DSG session from encrypted round-1 state and verify BitGo round-1 output from txRequest signature shares.
• Run WASM round 1 and build the round-2 signature share with user GPG.
• Encrypt round-2 session state with MPS_DSG_SIGNING_ROUND2_STATE adata.
• Support SJCL and v2 envelope decryption paths for round-1 session input.
• Add unit tests with signBitgoEddsaRound1 helper, happy path, and guards.

Ticket: WCI-477

[linear-code]

WCI-477

[Marzooqa]

Implementation is correct and consistent with the ECDSA pattern — correct use of verifyPeerMessageRoundOne, handleIncomingMessages, ed25519 GPG, and userMsgPayload carry-over in sessionPayload. Three minor nits inline; none blocking.

The base branch was changed.

[Marzooqa]

Implementation is correct — session restore, userMsgPayload carry-over, handleIncomingMessages round-2 output, and re-encryption with ROUND2_STATE adata all look good. Previous nits addressed. One minor dead code note: the assert(Array.isArray(transactions) && transactions.length === 1, ...) after getSignableHexAndDerivationPath is unreachable since that method already makes the same check — safe to remove in a follow-up.

[Marzooqa]

Implementation is correct — session restore, userMsgPayload carry-over, handleIncomingMessages round-2 output, and re-encryption with ROUND2_STATE adata all look good. Previous nits addressed. One minor dead code note: the assert(Array.isArray(transactions) && transactions.length === 1, ...) after getSignableHexAndDerivationPath is unreachable since that method already makes the same check — safe to remove in a follow-up.

[vibhavgo]

Implementation is correct — session restore, userMsgPayload carry-over, handleIncomingMessages round-2 output, and re-encryption with ROUND2_STATE adata all look good. Previous nits addressed. One minor dead code note: the assert(Array.isArray(transactions) && transactions.length === 1, ...) after getSignableHexAndDerivationPath is unreachable since that method already makes the same check — safe to remove in a follow-up.

From TypeScript narrowing perspective, the local assert is still useful, because TS does not understand that a previous helper call proved txRequest.transactions is defined.
src/bitgo/utils/tss/eddsa/eddsaMPCv2.ts:635:29 - error TS18048: 'transactions' is possibly 'undefined'.

635 const signatureShares = transactions[0].signatureShares;