fix(frontend): replace invalid "context" select field with "metadata" in threads.search (#2053)

* fix(frontend): replace invalid "context" select field with "metadata" in threads.search

The LangGraph API server does not support "context" as a select field for
threads/search, causing a 422 Unprocessable Entity error introduced by
commit 60e0abf (#1771).

- Replace "context" with "metadata" in the default select list
- Persist agent_name into thread metadata on creation so search results
  carry the agent identity
- Update pathOfThread() to fall back to metadata.agent_name when
  context is unavailable from search results
- Add regression tests for metadata-based agent routing

Fixes #2037

Made-with: Cursor

* fix: apply Copilot suggestions

* style: fix the lint error

frontend/src/core/threads/utils.test.ts

@@ -31,3 +31,24 @@ void test("uses provided context when pathOfThread is called with a thread id",
     "/workspace/agents/ops%20agent/chats/thread-123",
   );
 });
+
+void test("uses agent chat route when thread metadata has agent_name", () => {
+  assert.equal(
+    pathOfThread({
+      thread_id: "thread-456",
+      metadata: { agent_name: "coder" },
+    }),
+    "/workspace/agents/coder/chats/thread-456",
+  );
+});
+
+void test("prefers context.agent_name over metadata.agent_name", () => {
+  assert.equal(
+    pathOfThread({
+      thread_id: "thread-789",
+      context: { agent_name: "from-context" },
+      metadata: { agent_name: "from-metadata" },
+    }),
+    "/workspace/agents/from-context/chats/thread-789",
+  );
+});