feat(sdk-core): add createOfflineKeyGenRound2Share to EddsaMPCv2Utils

[bitgo-ai-agent-dev]

What

• Add createOfflineKeyGenRound2Share to EddsaMPCv2Utils in modules/sdk-core/src/bitgo/utils/tss/eddsa/eddsaMPCv2.ts
• The method decrypts the persisted round-1 session, restores the DKG state, verifies and deserializes all 3 incoming round-1 messages (own, counterParty, BitGo), calls dkg.handleIncomingMessages to produce ownMsg2, signs it with the GPG key, and returns { signedMsg2, encryptedRound2Session }
• Fix DKG.restoreSession in modules/sdk-lib-mpc/src/tss/eddsa-mps/dkg.ts to be async and load WASM before restoring state (mirrors DSG.restoreSession), which is required so that handleIncomingMessages works on a restored session
• Fix createOfflineKeyGenRound1Share (WCI-889) to call this.bitgo.encrypt/decrypt instead of non-existent encryptAsync/decryptAsync — BitGoBase.encrypt/decrypt already return Promise<string>

Why

• Required by the EdDSA MPCv2 external-signer AKM flow: AWM handlers must be able to drive DKG round 2 without holding decrypted key material in memory across API calls — the round-1 state is encrypted at the end of round 1 and restored here for round 2
• Completes the second of three signer methods needed for createKeychainsWithExternalSigner to function end-to-end

Test plan

• Unit tests: Round1 → Round2 produces valid { signedMsg2, encryptedRound2Session }
• Rejects tampered incoming messages (GPG signature verification fails)
• Rejects wrong walletPassphrase for session decryption
• Rejects non-BitGo public key when getEnv() returns 'prod' or 'test' (env requires BitGo pub key config)
• encryptedRound2Session is only decryptable with the correct walletPassphrase
• All 65 existing tests in eddsaMPCv2.ts continue to pass

Ticket: WCI-890

[linear-code]

WCI-890

[bitgo-ai-agent-dev]

The Audit API Spec CI failure is a pre-existing breakage on master (not caused by this PR).

The Generate merge base API spec job checks out master to build a baseline — and master currently fails to compile because WCI-889 merged encryptAsync/decryptAsync calls that don't exist on BitGoBase (lines 667 and 695 of eddsaMPCv2.ts).

This PR actually fixes that bug by replacing encryptAsync/decryptAsync with the correct encrypt/decrypt (which already return Promise<string>).

The BitGo SDK workflow (which tests our branch) passes cleanly on both CI runs — lint ✅, format ✅, unit tests ✅.