feat: 流式展示设计 Agent 对话

需求：设计 Agent 对话与确认生成的回复需要实时展示。

实现：统一通过 Agent Gateway 提交 Turn，接收并去重 assistant delta，以 canonical Workspace 收口，并修复跨项目旧请求回写竞态。

src/lib/image-workspace.ts

@@ -42,6 +42,10 @@ function createClientId(prefix: string): string {
   return `${prefix}-${id}`;
 }
 
+export function createImageWorkspaceTurnId(): string {
+  return createClientId('turn');
+}
+
 function getErrorStatus(error: unknown): number {
   if (error instanceof AppError && typeof error.details?.status === 'number') {
     return error.details.status;
@@ -120,13 +124,14 @@ export function sendImageWorkspaceMessage(
   workspaceId: string,
   expectedTurnRevision: number,
   message: string,
+  clientTurnId = createImageWorkspaceTurnId(),
 ): Promise<DesignWorkspace> {
   return requestData(
     `${IMAGE_WORKSPACE_API_PATH}/workspaces/${encodeURIComponent(workspaceId)}/messages`,
     {
       method: 'POST',
       body: JSON.stringify({
-        clientTurnId: createClientId('turn'),
+        clientTurnId,
         expectedTurnRevision,
         message: message.trim(),
         attachmentAssetIds: [],
@@ -139,13 +144,14 @@ export function confirmImageWorkspaceGeneration(
   workspaceId: string,
   expectedTurnRevision: number,
   quoteId: string,
+  clientTurnId = createImageWorkspaceTurnId(),
 ): Promise<DesignWorkspace> {
   return requestData(
     `${IMAGE_WORKSPACE_API_PATH}/workspaces/${encodeURIComponent(workspaceId)}/quotes/${encodeURIComponent(quoteId)}/confirm`,
     {
       method: 'POST',
       body: JSON.stringify({
-        clientTurnId: createClientId('turn'),
+        clientTurnId,
         expectedTurnRevision,
       }),
     },