feat(sdk-core): support preparedTransaction in TSS message signing

[ravibitgo]

Summary

wallet.signMessage() had no way to pass preparedTransaction — a field Canton's backend requires (as coinSpecific.preparedTransactionBase64) for both CANTON_SIGN_TRANSACTION and CANTON_SIGN_TOPOLOGY message standard types. Every field on the message-signing intent path is explicitly enumerated end-to-end (no generic passthrough bag), so any fresh Canton message-signing request created via the SDK was missing a mandatory field, regardless of whether the earlier commit-step server defect was fixed.

Without this, a client could only create a valid Canton message request via the raw POST /wallet/:id/msgrequests API directly, then use wallet.signMessage({ message: { txRequestId } }) only to sign it — wallet.signMessage() could never create one end-to-end on its own for Canton.

Changes

Threaded preparedTransaction?: string through the existing message-intent type chain, following the same pattern already established for Canton's cantonCommandParams on the transaction-intent side:

• Message (bitgo/baseCoin/iBaseCoin.ts) — public field on wallet.signMessage({ message: { preparedTransaction } }).
• IntentOptionsForMessage (bitgo/utils/tss/baseTypes.ts)
• PopulatedIntentForMessageSigning (bitgo/utils/tss/baseTypes.ts) — the shape POSTed to /wallet/:id/msgrequests.
• baseTSSUtils.buildSignMessageRequest — passes it into the built intent.
• wallet.buildSignMessageRequest — passes params.message.preparedTransaction.
• wallet.signMessageTss's inline intent construction (the fresh-request path taken when no txRequestId is given) — same, since it duplicates the intent shape instead of delegating to buildSignMessageRequest.

buildSignMessageRequestBase already spreads {...intent} into the POST body with no field whitelist, so no additional wiring was needed there.

Purely additive and optional — every other coin (SOL, ADA, Midnight, ETH typed-data) never sets this field, so it's undefined and omitted from their requests exactly as before.

Result

await wallet.signMessage({
  message: {
    messageRaw,
    messageStandardType: MessageStandardType.CANTON_SIGN_TOPOLOGY,
    preparedTransaction: '<base64 protobuf bytes>',
  },
  walletPassphrase,
});```

now creates the message txRequest (with the field Canton's backend requires) and signs it in one call, instead of requiring a raw API call to build the request first.

Test plan

- [x] New unit tests in modules/bitgo/test/v2/unit/wallet.ts (Message Signing suite):
- should include preparedTransaction in the msgrequests body when provided (via buildSignMessageRequest())
- should include preparedTransaction when signing a fresh message request(via signMessage(), no txRequestId) Both assert the intercepted POST body's intent.preparedTransaction.
- [x] npx tsc --noEmit on sdk-core — clean (no new errors)
- [ ] Manual: full Canton wallet.signMessage() flow against bitgo-test with preparedTransaction set, paired with the server-side commit-step

Ticket: CSHLD-1557

[linear-code]

CSHLD-1557