fix(server-party-co-managed): 修复死锁问题 - session_created 时立即 JoinSession

问题：
- 原来在 session_created 时只存储 token，等待 session_started
- 但 session_started 需要所有 N 方都 JoinSession 后才触发
- 这导致死锁：co-managed-party 永远收不到 session_started

修复：
- Phase 1 (session_created): 立即调用 JoinSession + 存储 session 信息
- Phase 2 (session_started): 执行 TSS 协议（超时从此时开始计算）

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

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

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

@@ -189,6 +189,7 @@ func main() {
 	eventHandler := createCoManagedSessionEventHandler(
 		ctx,
 		partyID,
+		messageRouter,
 		participateKeygenUC,
 	)
 
@@ -304,11 +305,12 @@ func startHTTPServer(cfg *config.Config) error {
 
 // createCoManagedSessionEventHandler creates a handler specifically for co_managed_keygen sessions
 // Two-phase event handling:
-// Phase 1 (session_created): Store join token and wait
+// Phase 1 (session_created): JoinSession immediately + store session info
 // Phase 2 (session_started): Execute TSS protocol (same timing as user clients receiving all_joined)
 func createCoManagedSessionEventHandler(
 	ctx context.Context,
 	partyID string,
+	messageRouter *grpcclient.MessageRouterClient,
 	participateKeygenUC *use_cases.ParticipateKeygenUseCase,
 ) func(*router.SessionEvent) {
 	return func(event *router.SessionEvent) {
@@ -350,7 +352,7 @@ func createCoManagedSessionEventHandler(
 				return
 			}
 
-			// Phase 1: Store session info and wait for session_started
+			// Phase 1: Get join token
 			joinToken, exists := event.JoinTokens[partyID]
 			if !exists {
 				logger.Error("No join token found for party in session_created",
@@ -359,6 +361,22 @@ func createCoManagedSessionEventHandler(
 				return
 			}
 
+			// 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)
+			joinCancel()
+			if err != nil {
+				logger.Error("Failed to join session",
+					zap.String("session_id", event.SessionId),
+					zap.String("party_id", partyID),
+					zap.Error(err))
+				return
+			}
+
+			logger.Info("Successfully joined session, waiting for session_started",
+				zap.String("session_id", event.SessionId),
+				zap.String("party_id", partyID))
+
 			// Store pending session for later use when session_started arrives
 			pendingSessionCache.Store(event.SessionId, &PendingSession{
 				SessionID:   sessionID,
@@ -367,10 +385,6 @@ func createCoManagedSessionEventHandler(
 				CreatedAt:   time.Now(),
 			})
 
-			logger.Info("Session created event received, waiting for session_started",
-				zap.String("session_id", event.SessionId),
-				zap.String("party_id", partyID))
-
 		case "session_started":
 			// Phase 2: All participants have joined, now execute TSS protocol
 			pendingSession, exists := pendingSessionCache.Get(event.SessionId)
@@ -386,6 +400,7 @@ func createCoManagedSessionEventHandler(
 				zap.String("party_id", partyID))
 
 			// Execute TSS keygen protocol in goroutine
+			// Timeout starts NOW (when session_started is received), not at session_created
 			go func() {
 				// 10 minute timeout for TSS protocol execution
 				participateCtx, cancel := context.WithTimeout(ctx, 10*time.Minute)