feat(sdk-core): getUserAndBackupSession + createKeychains retrofit wiring

[bitgo-ai-agent-dev]

What

• Added retrofit?: DecryptedRetrofitPayload to EddsaMPCv2Utils.createKeychains() params
• Added private getUserAndBackupSession(retrofit?) method that branches on the retrofit flag:
  • With retrofit: derives EddsaRetrofitData from MPCv1 key shares via getMpcV2RetrofitDataFromMpcV1Keys and constructs DKG instances with retrofit data
  • Without retrofit: constructs plain DKG instances (existing behavior unchanged)
• Replaced inline new EddsaMPSDkg.DKG(...) calls in createKeychains with getUserAndBackupSession
• Extended sendKeyGenerationRound1 / sendKeyGenerationRound1BySender payload types to allow walletId?: string
• Spread walletId into the R1 request body when params.retrofit?.walletId is present, so server-side isRound1RetrofitDKG() detection kicks in
• Unit tests for getUserAndBackupSession (both paths) and for walletId presence/absence in the captured R1 payload

Why

• EdDSA wallet retrofit (upgrading MPCv1 → MPCv2) requires the DKG ceremony to be seeded with the existing MPCv1 key material instead of fresh randomness, so the resulting MPCv2 keychain derives from the same root key that the wallet already uses
• The server identifies a retrofit DKG round by the presence of walletId in the R1 payload; without this wiring the server would treat every key-gen as a fresh creation
• Follows the identical pattern already implemented for ECDSA in ecdsaMPCv2.ts (line 639 getUserAndBackupSession, line 126-129 walletId spread)

Test plan

• EddsaMPCv2Utils.getUserAndBackupSession — returns plain DKG sessions when retrofit is undefined
• EddsaMPCv2Utils.getUserAndBackupSession — returns retrofit-seeded DKG sessions when retrofit payload is supplied
• EddsaMPCv2Utils.createKeychains with retrofit wiring — walletId present in R1 payload when retrofit provided
• EddsaMPCv2Utils.createKeychains with retrofit wiring — walletId absent from R1 payload when no retrofit
• All existing EddsaMPCv2Utils unit tests continue to pass

Stack

This PR is part 3 of 3 in a stacked series. Review and merge in order:

1. WCI-1261 — EddsaRetrofitData type + DKG retrofit constructor + getFirstMessage routing (base: master)
2. WCI-1263 — getMpcV2RetrofitDataFromMpcV1Keys for EdDSA (base: WCI-1261-EddsaRetrofitData-DKG-constructor)
3. This PR — getUserAndBackupSession + createKeychains retrofit wiring (base: WCI-1261-EddsaRetrofitData-DKG-constructor) ← you are here

Ticket: WCI-1264

[linear-code]

WCI-1264

[vibhavgo]

lgtm