fix(co-managed): 修复签名时使用错误 keyshare 的关键 bug

## 问题现象
2-of-3 服务器参与签名时 TSS 协议失败：
```
[TSS-SIGN] ERROR: failed to calculate Bob_mid or Bob_mid_wc
```

## 根本原因
服务器使用了错误的 keyshare：
- 签名会话的 keygen_session_id: c1e66501-bf6b-4d75-8c03-ba547ca82e1b
- 服务器实际加载的 keyshare: 01f2eb3b-e038-4806-b474-b131e6bf9d8e (most recent)
- 原因：main.go:449 传递了 KeygenSessionID: uuid.Nil，触发回退逻辑

## 修复内容

### 1. PendingSession 结构体添加 KeygenSessionID 字段 (33行)
```go
type PendingSession struct {
    SessionID       uuid.UUID
    JoinToken       string
    MessageHash     []byte
    KeygenSessionID uuid.UUID // 新增：从 JoinSession 获取的正确 keygen session ID
    ThresholdN      int
    ThresholdT      int
    SelectedParties []string
    CreatedAt       time.Time
}
```

### 2. session_created 阶段保存 keygen_session_id (395-419行)
**修改前**：
```go
_, err := messageRouter.JoinSession(joinCtx, sessionID, partyID, joinToken)
// ... 忽略返回值，后续使用 uuid.Nil
```

**修改后**：
```go
sessionInfo, err := messageRouter.JoinSession(joinCtx, sessionID, partyID, joinToken)
// ... 保存到 pendingSession
pendingSessionCache.Store(event.SessionId, &PendingSession{
    KeygenSessionID: sessionInfo.KeygenSessionID, // 保存正确的 keygen session ID
    // ...
})
```

### 3. session_started 阶段使用正确的 keygen_session_id (439-453行)
**修改前**：
```go
sessionInfo := &use_cases.SessionInfo{
    KeygenSessionID: uuid.Nil, // 错误：触发回退逻辑
}
```

**修改后**：
```go
sessionInfo := &use_cases.SessionInfo{
    KeygenSessionID: pendingSession.KeygenSessionID, // 正确：使用 JoinSession 返回的 ID
}
```

## 日志改进
- session_created: 记录 keygen_session_id (407行)
- session_started: 记录 keygen_session_id (442行)

## 测试计划
1. 重启 server-party-co-managed 服务
2. Android 客户端勾选"包含服务器备份"发起转账
3. 检查服务器日志：应该使用正确的 keygen_session_id，不再有 "Using most recent keyshare" 警告
4. 验证 TSS 签名协议成功完成

## 影响范围
- 仅影响 server-party-co-managed 的 2-of-3 签名功能
- 不影响 keygen 功能
- 不影响其他服务

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

backend/mpc-system/services/server-party-co-managed/cmd/server/main.go

@@ -32,6 +32,7 @@ type PendingSession struct {
 	SessionID       uuid.UUID
 	JoinToken       string
 	MessageHash     []byte
+	KeygenSessionID uuid.UUID // For sign sessions: the keygen session that created the keys
 	ThresholdN      int
 	ThresholdT      int
 	SelectedParties []string
@@ -392,7 +393,7 @@ func createCoManagedSessionEventHandler(
 
 			// Immediately call JoinSession (this is required to trigger session_started)
 			joinCtx, joinCancel := context.WithTimeout(ctx, 30*time.Second)
-			_, err := messageRouter.JoinSession(joinCtx, sessionID, partyID, joinToken)
+			sessionInfo, err := messageRouter.JoinSession(joinCtx, sessionID, partyID, joinToken)
 			joinCancel()
 			if err != nil {
 				logger.Error("Failed to join session",
@@ -404,13 +405,15 @@ func createCoManagedSessionEventHandler(
 
 			logger.Info("Successfully joined session, waiting for session_started",
 				zap.String("session_id", event.SessionId),
-				zap.String("party_id", partyID))
+				zap.String("party_id", partyID),
+				zap.String("keygen_session_id", sessionInfo.KeygenSessionID.String()))
 
 			// Store pending session for later use when session_started arrives
 			pendingSessionCache.Store(event.SessionId, &PendingSession{
 				SessionID:       sessionID,
 				JoinToken:       joinToken,
 				MessageHash:     event.MessageHash,
+				KeygenSessionID: sessionInfo.KeygenSessionID, // CRITICAL: Save the correct keygen session ID from JoinSession
 				ThresholdN:      int(event.ThresholdN),
 				ThresholdT:      int(event.ThresholdT),
 				SelectedParties: event.SelectedParties,
@@ -464,7 +467,8 @@ func createCoManagedSessionEventHandler(
 					// Execute signing protocol
 					logger.Info("Auto-participating in co_managed_sign session",
 						zap.String("session_id", event.SessionId),
-						zap.String("party_id", partyID))
+						zap.String("party_id", partyID),
+						zap.String("keygen_session_id", pendingSession.KeygenSessionID.String()))
 
 					sessionInfo := &use_cases.SessionInfo{
 						SessionID:        pendingSession.SessionID,
@@ -472,7 +476,7 @@ func createCoManagedSessionEventHandler(
 						ThresholdN:       int(event.ThresholdN),
 						ThresholdT:       int(event.ThresholdT),
 						MessageHash:      pendingSession.MessageHash,
-						KeygenSessionID:  uuid.Nil, // Use nil to trigger fallback logic (load most recent share)
+						KeygenSessionID:  pendingSession.KeygenSessionID, // CRITICAL: Use the correct keygen session ID from JoinSession
 						Participants:     participants,
 					}