feat(frontend): 支持 Roundtable 写文件流式预览与消息流聚合展示

功能说明：\n- 为写文件工具链增加流式预览状态模型（in-flight、chunk 归并、已完成）\n- 在消息面板中支持 write_file 流式 artifact 的动态插入/更新与完成态呈现\n- 优化 artifact 详情页的文件名与来源显示，兼容流式生成中文件路径\n- 在 Roundtable 工作流 step2 流程中注入与监听 write_file 事件，支持多阶段消息统一展示\n- 补充单测覆盖：streaming-write-preview、write-file-in-flight、streaming-artifacts 等关键路径

frontend-web/src/components/workspace/artifacts/artifact-file-detail.tsx

@@ -428,11 +428,15 @@ export function ArtifactFileDetail({
     if (streamPerfEnabled && isLiveStreaming) return raw;
     return desensitize(raw);
   }, [disableDesensitize, displayContent, isLiveStreaming, streamPerfEnabled]);
-  const artifactContentPending =
-    isCodeFile &&
-    !useWriteFileStreaming &&
-    content === undefined &&
-    isArtifactContentLoading;
+  const artifactContentPending =
+    isCodeFile &&
+    !useWriteFileStreaming &&
+    content === undefined &&
+    isArtifactContentLoading;
+  // `write_file` 的 path 会先于正文抵达。它仍是正常的流式过程，不能显示成一块
+  // 空白编辑器，更不能误导用户去点击重试真实文件；下一帧内容到达后会自然切换为代码视图。
+  const streamingWriteContentPending =
+    useWriteFileStreaming && writeFileInFlight && !content;
   const artifactContentUnavailable =
     isCodeFile &&
     !useWriteFileStreaming &&
@@ -1766,10 +1770,10 @@ export function ArtifactFileDetail({
                 activeCursor={isApplyingEdit}
               />
             )}
-        {artifactContentPending && viewMode === "code" && (
-          <div className="text-muted-foreground flex size-full flex-col items-center justify-center gap-3 text-sm">
-            <LoaderIcon aria-hidden="true" className="size-5 animate-spin" />
-            正在加载文档内容…
+        {(artifactContentPending || streamingWriteContentPending) && viewMode === "code" && (
+          <div className="text-muted-foreground flex size-full flex-col items-center justify-center gap-3 text-sm">
+            <LoaderIcon aria-hidden="true" className="size-5 animate-spin" />
+            {streamingWriteContentPending ? "正在接收文件内容…" : "正在加载文档内容…"}
           </div>
         )}
         {artifactContentUnavailable && viewMode === "code" && (
@@ -1787,7 +1791,7 @@ export function ArtifactFileDetail({
             </button>
           </div>
         )}
-        {isCodeFile && viewMode === "code" && !artifactContentPending && !artifactContentUnavailable && (
+        {isCodeFile && viewMode === "code" && !artifactContentPending && !streamingWriteContentPending && !artifactContentUnavailable && (
           <div className="size-full">
             <CodeEditor
               className="size-full resize-none rounded-none border-none bg-transparent"

frontend-web/src/components/workspace/messages/message-group.tsx

@@ -40,9 +40,10 @@ import {
   DialogTitle,
 } from "@/components/ui/dialog";
 import {
-  buildWriteFileVirtualUrl,
-  hasCompleteArtifactExtension,
-  languageOfStreamingArtifact,
+  buildWriteFileVirtualUrl,
+  hasCompleteArtifactExtension,
+  isAutoPreviewableArtifactPath,
+  languageOfStreamingArtifact,
   normalizeStreamingArtifactPath,
   provisionalWriteFilePath,
 } from "@/core/artifacts/streaming-write-preview";
@@ -802,9 +803,10 @@ function ArtifactWriteToolCall({
       !artifactPath ||
       !previewKey ||
       !isLoading ||
-      result !== undefined ||
-      !sourcePath ||
-      !languageOfStreamingArtifact(sourcePath)
+      result !== undefined ||
+      !sourcePath ||
+      !languageOfStreamingArtifact(sourcePath) ||
+      !isAutoPreviewableArtifactPath(sourcePath)
     ) {
       return;
     }