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.

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

backend/packages/harness/deerflow/config/paths.py

@@ -1,5 +1,6 @@
 import os
 import re
+import shutil
 from pathlib import Path
 
 # Virtual path prefix seen by agents inside the sandbox
@@ -155,6 +156,15 @@ class Paths:
             d.mkdir(parents=True, exist_ok=True)
             d.chmod(0o777)
 
+    def delete_thread_dir(self, thread_id: str) -> None:
+        """Delete all persisted data for a thread.
+
+        The operation is idempotent: missing thread directories are ignored.
+        """
+        thread_dir = self.thread_dir(thread_id)
+        if thread_dir.exists():
+            shutil.rmtree(thread_dir)
+
     def resolve_virtual_path(self, thread_id: str, virtual_path: str) -> Path:
         """Resolve a sandbox virtual path to the actual host filesystem path.