fix(threads): clean up local thread data after thread deletion (#1262)

* fix(threads): clean up local thread data after thread deletion

Delete DeerFlow-managed thread directories after the web UI removes a LangGraph thread.
This keeps local thread data in sync with conversation deletion and adds regression coverage for the cleanup flow.

* fix(threads): address thread cleanup review feedback

Encode thread cleanup URLs in the web client, keep cache updates explicit when no thread search data is cached, and return a generic 500 response from the cleanup endpoint while documenting the sanitized error behavior.

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/threads/hooks.ts

@@ -8,6 +8,7 @@ import { toast } from "sonner";
 import type { PromptInputMessage } from "@/components/ai-elements/prompt-input";
 
 import { getAPIClient } from "../api";
+import { getBackendBaseURL } from "../config";
 import { useI18n } from "../i18n/hooks";
 import type { FileInMessage } from "../messages/utils";
 import type { LocalSettings } from "../settings";
@@ -481,6 +482,20 @@ export function useDeleteThread() {
   return useMutation({
     mutationFn: async ({ threadId }: { threadId: string }) => {
       await apiClient.threads.delete(threadId);
+
+      const response = await fetch(
+        `${getBackendBaseURL()}/api/threads/${encodeURIComponent(threadId)}`,
+        {
+          method: "DELETE",
+        },
+      );
+
+      if (!response.ok) {
+        const error = await response
+          .json()
+          .catch(() => ({ detail: "Failed to delete local thread data." }));
+        throw new Error(error.detail ?? "Failed to delete local thread data.");
+      }
     },
     onSuccess(_, { threadId }) {
       queryClient.setQueriesData(
@@ -488,11 +503,17 @@ export function useDeleteThread() {
           queryKey: ["threads", "search"],
           exact: false,
         },
-        (oldData: Array<AgentThread>) => {
+        (oldData: Array<AgentThread> | undefined) => {
+          if (oldData == null) {
+            return oldData;
+          }
           return oldData.filter((t) => t.thread_id !== threadId);
         },
       );
     },
+    onSettled() {
+      void queryClient.invalidateQueries({ queryKey: ["threads", "search"] });
+    },
   });
 }