feat(sdk-core): add webauthnInfo param to AddKeychainOptions for atomic passkey attachment

The backend POST /api/v2/:coin/key handler reads `req.body.webauthnInfo` (single object)
to atomically attach a passkey during key creation, but the SDK only sent
`webauthnDevices` (array) which the backend ignores. This caused passkey attachment to
require a separate best-effort PUT call that could fail, leaving wallets in an
inconsistent state.

Add `webauthnInfo` field to `AddKeychainOptions` and pass it through in `keychains.add()`
so callers can atomically link a passkey to a keychain during creation.

Deprecate `webauthnDevices` on `AddKeychainOptions` since the backend never reads it
on the POST path.

WCN-127

Author: rajangarg047

modules/sdk-core/src/bitgo/keychain/iKeychains.ts

@@ -141,8 +141,13 @@ export interface AddKeychainOptions {
   // indicates if the key is MPCv2 or not
   isMPCv2?: boolean;
   coinSpecific?: { [coinName: string]: unknown };
-  /** WebAuthn devices that have an additional encrypted copy of the private key, keyed by PRF-derived passphrases. */
+  /** WebAuthn devices that have an additional encrypted copy of the private key, keyed by PRF-derived passphrases.
+   *  @deprecated Use {@link webauthnInfo} instead — the backend reads `webauthnInfo` (single object), not `webauthnDevices`. */
   webauthnDevices?: WebauthnInfo[];
+  /** Single webauthn device to atomically attach during key creation.
+   *  Sent as `webauthnInfo` in the POST /key body so the backend can link
+   *  the passkey to the keychain in the same request. */
+  webauthnInfo?: WebauthnInfo;
 }
 
 export interface ApiKeyShare {

modules/sdk-core/src/bitgo/keychain/keychains.ts

@@ -307,6 +307,7 @@ export class Keychains implements IKeychains {
         isMPCv2: params.isMPCv2,
         coinSpecific: params.coinSpecific,
         webauthnDevices: params.webauthnDevices,
+        webauthnInfo: params.webauthnInfo,
       })
       .result();
   }