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。

tests/unit/image-workspace-api.test.ts

@@ -80,7 +80,12 @@ describe('AI design renderer API boundary', () => {
   });
 
   it('sends conversation turns with the current turn revision and no provider settings', async () => {
-    await sendImageWorkspaceMessage('workspace/one', 4, '  继续调整构图  ');
+    await sendImageWorkspaceMessage(
+      'workspace/one',
+      4,
+      '  继续调整构图  ',
+      'turn-client-1',
+    );
 
     const [path, init] = hostApiFetchMock.mock.calls[0];
     expect(path).toBe('/api/works/image-workspace/workspaces/workspace%2Fone/messages');
@@ -89,7 +94,7 @@ describe('AI design renderer API boundary', () => {
       expectedTurnRevision: 4,
       message: '继续调整构图',
       attachmentAssetIds: [],
-      clientTurnId: expect.stringMatching(/^turn-/),
+      clientTurnId: 'turn-client-1',
     });
     expect(String(init?.body)).not.toMatch(/model|resolution|outputCount/);
   });