feat(sdk-coin-xdc): add signXdcKycMessage utility

[bitgo-ai-agent-dev]

What

• Add buildXdcKycMessage(params) — constructs the canonical KYC message string
  "[XDCmaster KYC <ISO-timestamp>] Upload KYC for <account>" expected by the
  XinFin masternode IPFS API.
• Add signXdcKycMessage(wallet, account, walletPassphrase) — generates a fresh
  timestamp, builds the message, signs it via wallet.signMessage() with
  MessageStandardType.EIP191, and returns { kycAccount, kycMessage, kycSignature }.
• Export both functions and related types from modules/sdk-coin-xdc/src/lib/index.ts.
• 5 unit tests covering: correct format string, timestamp non-determinism, correct
  signMessage call params, correct return value mapping, and error propagation.

Why

• The XinFin masternode IPFS API (/api/ipfs/addKYC) now requires three signed-message
  headers (x-kyc-account, x-kyc-message, x-kyc-signature) to prove wallet
  ownership before accepting KYC file uploads.
• The staking-service and frontend (SI-922, SI-923) need a ready-made utility to
  build and sign this message via the existing TSS wallet.signMessage() flow,
  without requiring any changes to wallet-platform (XDC already has
  supportsMessageSigning() = true).

Test plan

• buildXdcKycMessage — correct format string with account + timestamp
• buildXdcKycMessage — different timestamps produce different strings
• signXdcKycMessage — calls wallet.signMessage with correct messageRaw
  and messageStandardType: MessageStandardType.EIP191
• signXdcKycMessage — returns kycAccount, kycMessage, kycSignature
  mapped correctly from SignedMessage
• signXdcKycMessage — propagates error if wallet.signMessage throws
• All 62 existing XDC unit tests still pass

Ticket: SI-921

[linear-code]

SI-921

[Doddanna17]

Reviewed against SI-921. Implementation is correct and complete.

• Message format matches XinFin foundation spec exactly: [XDCmaster KYC <ISO-timestamp>] Upload KYC for <account>
• MessageStandardType.EIP191 enum over raw string — good call
• Timestamp generated inside signXdcKycMessage so kycMessage and the signed payload are always in sync
• Standalone utility pattern (not on the Xdc class) matches the signAccountBasedMidnightClaimMessages precedent
• All 5 test cases from the ticket are covered, including error propagation and the kycMessage/messageRaw consistency assertion
• Zero deletions — no regressions to existing XDC tests

SI-921 complete. Unblocks SI-922 (staking-service) and SI-923 (frontend).

[abhijit0943]

LG