fix(android): 修复参与者列表在keygen开始后继续增加的bug

问题: 发起者端参与方显示4/3（2-of-3 MPC）
原因: session_started事件后仍继续处理participant_joined事件

修复方案:
1. 在participant_joined处理中检查sessionStatus
2. 如果已是IN_PROGRESS或COMPLETED则忽略新的participant_joined事件
3. session_started时只补全不完整的列表，不覆盖

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

backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt

@@ -343,8 +343,10 @@ class MainViewModel @Inject constructor(
                     val currentSessionId = _currentSessionId.value
                     if (currentSessionId != null && event.sessionId == currentSessionId) {
                         android.util.Log.d("MainViewModel", "Session started event for keygen initiator, triggering keygen")
-                        // Initialize participant list with all N parties (keygen requires all parties)
+                        // Ensure participant list has exactly N parties (fill if incomplete, don't add more)
-                        _sessionParticipants.value = (1..event.thresholdN).map { "参与方 $it" }
+                        if (_sessionParticipants.value.size < event.thresholdN) {
+                            _sessionParticipants.value = (1..event.thresholdN).map { "参与方 $it" }
+                        }
                         viewModelScope.launch {
                             startKeygenAsInitiator(
                                 sessionId = currentSessionId,
@@ -359,6 +361,10 @@ class MainViewModel @Inject constructor(
                     val joinKeygenInfo = pendingJoinKeygenInfo
                     if (joinKeygenInfo != null && event.sessionId == joinKeygenInfo.sessionId) {
                         android.util.Log.d("MainViewModel", "Session started event for keygen joiner, triggering keygen")
+                        // Ensure participant list has exactly N parties
+                        if (_joinKeygenParticipants.value.size < event.thresholdN) {
+                            _joinKeygenParticipants.value = (1..event.thresholdN).map { "参与方 $it" }
+                        }
                         startKeygenAsJoiner()
                     }
 
@@ -366,8 +372,10 @@ class MainViewModel @Inject constructor(
                     val joinSignInfo = pendingJoinSignInfo
                     if (joinSignInfo != null && event.sessionId == joinSignInfo.sessionId) {
                         android.util.Log.d("MainViewModel", "Session started event for sign joiner, triggering sign")
-                        // Initialize participant list with T parties (sign requires T parties)
+                        // Ensure participant list has exactly T parties
-                        _coSignParticipants.value = (1..event.thresholdT).map { "参与方 $it" }
+                        if (_coSignParticipants.value.size < event.thresholdT) {
+                            _coSignParticipants.value = (1..event.thresholdT).map { "参与方 $it" }
+                        }
                         startSignAsJoiner()
                     }
 
@@ -376,8 +384,10 @@ class MainViewModel @Inject constructor(
                     android.util.Log.d("MainViewModel", "Checking for sign initiator: signSessionId=$signSessionId, eventSessionId=${event.sessionId}")
                     if (signSessionId != null && event.sessionId == signSessionId) {
                         android.util.Log.d("MainViewModel", "Session started event for sign initiator, triggering sign")
-                        // Initialize participant list with T parties (sign requires T parties)
+                        // Ensure participant list has exactly T parties
-                        _signParticipants.value = (1..event.thresholdT).map { "参与方 $it" }
+                        if (_signParticipants.value.size < event.thresholdT) {
+                            _signParticipants.value = (1..event.thresholdT).map { "参与方 $it" }
+                        }
                         startSignAsInitiator(event.selectedParties)
                     } else {
                         android.util.Log.d("MainViewModel", "NOT triggering sign initiator: signSessionId=$signSessionId, pendingSignInitiatorInfo=${pendingSignInitiatorInfo?.sessionId}")
@@ -386,6 +396,14 @@ class MainViewModel @Inject constructor(
                 "party_joined", "participant_joined" -> {
                     android.util.Log.d("MainViewModel", "Processing participant_joined event...")
 
+                    // Don't add participants if keygen/sign has already started
+                    // This prevents duplicate additions after session_started event
+                    val currentStatus = repository.sessionStatus.value
+                    if (currentStatus == SessionStatus.IN_PROGRESS || currentStatus == SessionStatus.COMPLETED) {
+                        android.util.Log.d("MainViewModel", "  Session already in progress/completed, ignoring participant_joined")
+                        return@collect
+                    }
+
                     // Update participant count for initiator's CreateWallet screen
                     val currentSessionId = _currentSessionId.value
                     android.util.Log.d("MainViewModel", "  Checking for initiator: currentSessionId=$currentSessionId, eventSessionId=${event.sessionId}")