fix(server-party): 支持 co_managed_keygen 会话类型

server-party 的 ParticipateKeygenUseCase 现在同时接受 "keygen" 和
"co_managed_keygen" 两种会话类型，使 persistent party 能够正确参与
共管钱包的密钥生成流程。

🤖 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/application/use_cases/participate_keygen.go

@@ -110,7 +110,8 @@ func (uc *ParticipateKeygenUseCase) Execute(
 		return nil, err
 	}
 
-	if sessionInfo.SessionType != "keygen" {
+	// Accept both "keygen" and "co_managed_keygen" session types
+	if sessionInfo.SessionType != "keygen" && sessionInfo.SessionType != "co_managed_keygen" {
 		return nil, ErrInvalidSession
 	}