fix(storage): 修复 multi-step 对话产生多个 assistant 消息问题

- 同一轮对话的后续 assistant 消息追加 Parts 到现有消息
- 修复 AI SDK tool-call 参数字段名（input 而非 args）
- 修复 setToolCompleted/setToolError 中 discriminated union 类型访问

packages/core/src/session/manager.ts

@@ -397,39 +397,55 @@ export class SessionManager {
         toolCallPartIds.clear();
 
       } else if (message.role === 'assistant') {
-        // Assistant 消息
-        const messageInfo = await MessageStorage.create(sessionId, 'assistant', {
-          parentId: currentUserMsgId ?? undefined,
-        });
-        currentAssistantMsgId = messageInfo.id;
-        const partIds: string[] = [];
+        // Assistant 消息：如果当前轮次已有 assistant 消息，则追加 Parts
+        let messageId: string;
+        let existingPartIds: string[] = [];
+
+        if (currentAssistantMsgId) {
+          // 同一轮对话的后续 assistant 消息，追加到现有消息
+          messageId = currentAssistantMsgId;
+          const existingMsg = await MessageStorage.get(sessionId, messageId);
+          existingPartIds = existingMsg?.partIds ?? [];
+        } else {
+          // 新的 assistant 消息
+          const messageInfo = await MessageStorage.create(sessionId, 'assistant', {
+            parentId: currentUserMsgId ?? undefined,
+          });
+          messageId = messageInfo.id;
+          currentAssistantMsgId = messageId;
+        }
+
+        const newPartIds: string[] = [];
 
         if (typeof message.content === 'string') {
-          const part = await PartStorage.createText(messageInfo.id, message.content);
-          partIds.push(part.id);
+          const part = await PartStorage.createText(messageId, message.content);
+          newPartIds.push(part.id);
         } else if (Array.isArray(message.content)) {
           for (const item of message.content) {
             const itemType = (item as { type: string }).type;
             if (itemType === 'text') {
-              const part = await PartStorage.createText(messageInfo.id, (item as { text: string }).text);
-              partIds.push(part.id);
+              const part = await PartStorage.createText(messageId, (item as { text: string }).text);
+              newPartIds.push(part.id);
             } else if (itemType === 'tool-call') {
-              const toolCall = item as unknown as { toolCallId: string; toolName: string; args: Record<string, unknown> };
+              // AI SDK 的 tool-call 使用 input 字段存储参数（不是 args）
+              const toolCall = item as unknown as { toolCallId: string; toolName: string; input: Record<string, unknown> };
               // 创建 running 状态的工具 Part
               const part = await PartStorage.createToolRunning(
-                messageInfo.id,
+                messageId,
                 toolCall.toolCallId,
                 toolCall.toolName,
-                toolCall.args ?? {}
+                (toolCall.input as Record<string, unknown>) ?? {}
               );
-              partIds.push(part.id);
+              newPartIds.push(part.id);
               toolCallPartIds.set(toolCall.toolCallId, part.id);
             }
           }
         }
 
-        if (partIds.length > 0) {
-          await MessageStorage.update(sessionId, messageInfo.id, { partIds });
+        if (newPartIds.length > 0) {
+          // 合并已有的和新的 partIds
+          const allPartIds = [...existingPartIds, ...newPartIds];
+          await MessageStorage.update(sessionId, messageId, { partIds: allPartIds });
         }
 
       } else if (message.role === 'tool' && currentAssistantMsgId) {

packages/core/src/session/storage/part.ts

@@ -98,7 +98,11 @@ export async function setToolCompleted(
 ): Promise<ToolPart> {
   // 先获取当前 part 以获取 input
   const part = await get(messageId, partId) as ToolPart | null;
-  const input = part?.state.status !== 'pending' ? part?.state.input : {};
+  // 从 running 状态获取 input（需要类型断言因为 discriminated union）
+  const state = part?.state;
+  const input = state && state.status !== 'pending'
+    ? (state as { input: Record<string, unknown> }).input
+    : {};
 
   return updateToolState(messageId, partId, {
     status: 'completed',
@@ -119,7 +123,11 @@ export async function setToolError(
 ): Promise<ToolPart> {
   // 先获取当前 part 以获取 input
   const part = await get(messageId, partId) as ToolPart | null;
-  const input = part?.state.status !== 'pending' ? part?.state.input : {};
+  // 从 running 状态获取 input（需要类型断言因为 discriminated union）
+  const state = part?.state;
+  const input = state && state.status !== 'pending'
+    ? (state as { input: Record<string, unknown> }).input
+    : {};
 
   return updateToolState(messageId, partId, {
     status: 'error',