fix(frontend): show filename for presented artifacts missing from thread.values.artifacts (#3198)

Closes #3192

Root cause
----------
The artifact preview header is driven by a Radix Select whose <SelectValue>
renders the label of the <SelectItem> matching the current value. The option
list was built solely from `artifacts` in ArtifactsContext, which is only
synced from `thread.values.artifacts` (chat-box.tsx). Artifacts surfaced via
the message-layer `present_files` tool call are never written back to
`thread.values.artifacts`, so when such a file is selected its filepath has
no matching <SelectItem>. Radix then renders an empty trigger and the header
filename appears blank, even though the preview body loads correctly.

Fix
---
Compute `artifactOptions` as a defensive union: if the currently selected
filepath is missing from `artifacts`, prepend it so a matching <SelectItem>
always exists. This keeps the header label in sync with the active file
without changing context semantics or coupling the UI to a future
auto-discovery mechanism (see existing TODO in chat-box.tsx).

Tests
-----
Add an e2e case that mocks a thread whose only artifact is delivered via
`present_files` (thread.values.artifacts = []) and asserts both the header
title and preview content render. All 21 e2e tests pass.

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

@@ -84,6 +84,13 @@ export function ArtifactFileDetail({
     }
     return filepathFromProps;
   }, [filepathFromProps, isWriteFile]);
+  const artifactOptions = useMemo(() => {
+    const list = artifacts ?? [];
+    if (list.includes(filepath)) {
+      return list;
+    }
+    return [filepath, ...list];
+  }, [artifacts, filepath]);
   const isSkillFile = useMemo(() => {
     return filepath.endsWith(".skill");
   }, [filepath]);
@@ -178,9 +185,9 @@ export function ArtifactFileDetail({
                 </SelectTrigger>
                 <SelectContent className="select-none">
                   <SelectGroup>
-                    {(artifacts ?? []).map((filepath) => (
-                      <SelectItem key={filepath} value={filepath}>
-                        {getFileName(filepath)}
+                    {artifactOptions.map((option) => (
+                      <SelectItem key={option} value={option}>
+                        {getFileName(option)}
                       </SelectItem>
                     ))}
                   </SelectGroup>

frontend/tests/e2e/artifact-preview.spec.ts

@@ -5,11 +5,13 @@ import { mockLangGraphAPI } from "./utils/mock-api";
 const ARTIFACT_PATH = "/artifact-fixtures/report.html";
 const MARKDOWN_ARTIFACT_PATH = "/artifact-fixtures/report.md";
 const JSON_ARTIFACT_PATH = "/artifact-fixtures/report.json";
+const PRESENTED_ARTIFACT_PATH = "/mnt/user-data/outputs/presented-report.md";
 const IN_PROGRESS_THREAD_ID = "00000000-0000-0000-0000-000000003119";
 const COMPLETE_THREAD_ID = "00000000-0000-0000-0000-000000003120";
 const MARKDOWN_THREAD_ID = "00000000-0000-0000-0000-000000003121";
 const MARKDOWN_ANCHOR_THREAD_ID = "00000000-0000-0000-0000-000000003123";
 const JSON_THREAD_ID = "00000000-0000-0000-0000-000000003122";
+const PRESENTED_THREAD_ID = "00000000-0000-0000-0000-000000003123";
 
 function writeFileMessages({
   path = ARTIFACT_PATH,
@@ -57,6 +59,30 @@ function writeFileMessages({
   return messages;
 }
 
+function presentFilesMessages() {
+  return [
+    {
+      type: "human",
+      id: "msg-human-present-file",
+      content: [{ type: "text", text: "Create a markdown report" }],
+    },
+    {
+      type: "ai",
+      id: "msg-ai-present-file",
+      content: "The report has been written. Now let me present the file.",
+      tool_calls: [
+        {
+          id: "present-file-artifact",
+          name: "present_files",
+          args: {
+            filepaths: [PRESENTED_ARTIFACT_PATH],
+          },
+        },
+      ],
+    },
+  ];
+}
+
 test.describe("Artifact preview stability", () => {
   test("renders preview iframe for an in-progress write artifact", async ({
     page,
@@ -235,4 +261,45 @@ test.describe("Artifact preview stability", () => {
       artifactsPanel.getByText('"中文字段": "测试内容"'),
     ).toBeVisible();
   });
+
+  test("shows the title for a presented artifact missing from thread artifacts", async ({
+    page,
+  }) => {
+    mockLangGraphAPI(page, {
+      threads: [
+        {
+          thread_id: PRESENTED_THREAD_ID,
+          title: "Presented artifact title fallback",
+          messages: presentFilesMessages(),
+          artifacts: [],
+        },
+      ],
+    });
+    await page.route(
+      `**/api/threads/${PRESENTED_THREAD_ID}/artifacts/mnt/user-data/outputs/presented-report.md`,
+      (route) =>
+        route.fulfill({
+          status: 200,
+          contentType: "text/markdown",
+          body: "# Presented Report\n\nGenerated content",
+        }),
+    );
+
+    await page.goto(`/workspace/chats/${PRESENTED_THREAD_ID}`);
+
+    // The file card in the message list shows the basename only.
+    await expect(page.getByText("presented-report.md")).toBeVisible({
+      timeout: 15_000,
+    });
+    await page.getByText("presented-report.md").first().click();
+
+    const artifactsPanel = page.locator("#artifacts");
+
+    // 1. Header title should fall back to the current filepath even though
+    //    thread.values.artifacts is empty.
+    await expect(artifactsPanel.getByText("presented-report.md")).toBeVisible();
+
+    // 2. Preview content should still render correctly.
+    await expect(artifactsPanel.getByText("Presented Report")).toBeVisible();
+  });
 });