fix(co-managed): 使用数据库中的 PartyIndex 而非循环索引

问题:
- server-party-co-managed 在构建参与者列表时使用循环索引 i
- 导致 PartyIndex 映射错误: map[0:0 1:1] (错误)
- 应为: map[0:0 2:1] (Android 的 keygen index 是 2, 不是 1)
- TSS 协议因索引错误失败: "failed to calculate Bob_mid or Bob_mid_wc"

根本原因:
- event.SelectedParties 只包含 party ID, 不包含 PartyIndex
- 使用循环索引是假设,不是来自数据库的实际数据

解决方案:
1. PendingSession 添加 Participants 字段
2. 存储 JoinSession 返回的 sessionInfo.Participants
   - JoinSession 从数据库查询并返回所有参与方的正确 PartyIndex
3. session_started 时直接使用存储的 participants
   - 不再从 event.SelectedParties 构建

关键变更:
- PendingSession.Participants: 保存来自数据库的正确索引
- 移除循环构建逻辑: 不再假设 PartyIndex = loop index
- 数据来源: 数据库 (JoinSession response) → 缓存 → 使用

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

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

@@ -36,6 +36,7 @@ type PendingSession struct {
 	ThresholdN      int
 	ThresholdT      int
 	SelectedParties []string
+	Participants    []use_cases.ParticipantInfo // CRITICAL: Correct PartyIndex from database (via JoinSession)
 	CreatedAt       time.Time
 }
 
@@ -417,6 +418,7 @@ func createCoManagedSessionEventHandler(
 				ThresholdN:      int(event.ThresholdN),
 				ThresholdT:      int(event.ThresholdT),
 				SelectedParties: event.SelectedParties,
+				Participants:    sessionInfo.Participants, // CRITICAL: Save participants with correct PartyIndex from database
 				CreatedAt:       time.Now(),
 			})
 
@@ -450,18 +452,10 @@ func createCoManagedSessionEventHandler(
 				participateCtx, cancel := context.WithTimeout(ctx, 10*time.Minute)
 				defer cancel()
 
-				// Build SessionInfo from session_started event (NOT from pendingSession cache)
-				// session_started event contains ALL participants who have joined,
-				// including external parties that joined dynamically after session_created
-				// Note: We already called JoinSession in session_created phase,
-				// so we use ExecuteWithSessionInfo to skip the duplicate JoinSession call
-				participants := make([]use_cases.ParticipantInfo, len(event.SelectedParties))
-				for i, p := range event.SelectedParties {
-					participants[i] = use_cases.ParticipantInfo{
-						PartyID:    p,
-						PartyIndex: i,
-					}
-				}
+				// CRITICAL: Use participants from pendingSession (which came from JoinSession response)
+				// These contain the correct PartyIndex values from the database, NOT loop indices
+				// The JoinSession response already includes all participants with their assigned indices
+				participants := pendingSession.Participants
 
 				if isSignSession {
 					// Execute signing protocol