fix(mpc-service): 规范化 messageHash 去掉 0x 前缀

mpc-system 期望纯 hex 字符串（不带 0x 前缀），
blockchain-service 发送的 messageHash 带有 0x 前缀导致 400 错误

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/mpc-service/src/application/event-handlers/signing-requested.handler.ts

@@ -47,11 +47,18 @@ export class SigningRequestedHandler implements OnModuleInit {
 
     this.logger.log(`Processing signing request: userId=${userId}, username=${username}, sessionId=${sessionId}, source=${source}`);
 
+    // 规范化 messageHash: mpc-system 期望纯 hex 字符串（不带 0x 前缀）
+    const normalizedMessageHash = messageHash?.startsWith('0x')
+      ? messageHash.slice(2)
+      : messageHash;
+
+    this.logger.log(`Normalized messageHash: ${normalizedMessageHash?.slice(0, 16)}...`);
+
     try {
       // Step 1: Create signing session via mpc-system
       const createResult = await this.mpcCoordinator.createSigningSession({
         username,
-        messageHash,
+        messageHash: normalizedMessageHash,
         userShare,
       });