fix(android): 使用同步标志修复参与方显示4/3的竞态条件bug

问题根因:
- 之前使用异步的 sessionStatus 检查来防止 participant_joined 事件
  在 session_started 之后继续添加参与方
- 但 sessionStatus 是通过 StateFlow 异步更新的，检查时状态可能还未更新
- 导致 participant_joined 事件仍能添加额外的参与方，显示4/3而非3/3

解决方案:
- 添加同步标志 sessionStartedForSession: String?
- 在 session_started 处理器的最开始同步设置此标志
- 在 participant_joined 处理器中检查此标志，而非异步状态
- 由于回调是顺序处理的，这种方式100%可靠

修改内容:
1. 添加 sessionStartedForSession 私有成员变量
2. 在 session_started 事件处理开始时立即设置标志
3. 在 participant_joined 事件处理开始时检查标志
4. 在所有 reset 方法中重置标志:
   - resetSessionState()
   - resetJoinKeygenState()
   - resetCoSignState()
   - resetTransferState()

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

@@ -45,6 +45,11 @@ class MainViewModel @Inject constructor(
     private val _hasEnteredSession = MutableStateFlow(false)
     val hasEnteredSession: StateFlow<Boolean> = _hasEnteredSession.asStateFlow()
 
+    // Synchronous flag to prevent participant_joined from adding duplicates after session_started
+    // This is set immediately (synchronously) when session_started is processed, ensuring
+    // any subsequent participant_joined events in the same callback queue will see the flag
+    private var sessionStartedForSession: String? = null
+
     init {
         // Start initialization on app launch
         checkAllServices()
@@ -339,6 +344,12 @@ class MainViewModel @Inject constructor(
 
             when (event.eventType) {
                 "session_started" -> {
+                    // CRITICAL: Set flag immediately (synchronously) to prevent subsequent
+                    // participant_joined events from adding duplicates. This must be the
+                    // first line before any async operations.
+                    sessionStartedForSession = event.sessionId
+                    android.util.Log.d("MainViewModel", "Session started flag set for: ${event.sessionId}")
+
                     // Check if this is for keygen initiator (CreateWallet)
                     val currentSessionId = _currentSessionId.value
                     if (currentSessionId != null && event.sessionId == currentSessionId) {
@@ -396,11 +407,12 @@ 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")
+                    // CRITICAL: Check synchronous flag first - if session_started was already
+                    // processed for this session, don't add more participants
+                    // This is 100% reliable because the flag is set synchronously in session_started
+                    // handler before any async operations, and callbacks are processed sequentially
+                    if (sessionStartedForSession == event.sessionId) {
+                        android.util.Log.d("MainViewModel", "  Session already started for ${event.sessionId}, ignoring participant_joined")
                         return@setSessionEventCallback
                     }
 
@@ -548,6 +560,8 @@ class MainViewModel @Inject constructor(
         _publicKey.value = null
         _createdInviteCode.value = null
         _hasEnteredSession.value = false
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }
@@ -740,6 +754,8 @@ class MainViewModel @Inject constructor(
         pendingJoinToken = ""
         pendingPassword = ""
         pendingJoinKeygenInfo = null
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }
@@ -925,6 +941,8 @@ class MainViewModel @Inject constructor(
         pendingCoSignInviteCode = ""
         pendingCoSignJoinToken = ""
         pendingJoinSignInfo = null
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }
@@ -1671,6 +1689,8 @@ class MainViewModel @Inject constructor(
         _signature.value = null
         _txHash.value = null
         pendingSignInitiatorInfo = null
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }