fix(copilot): use _LOCAL_TOOL_RESULT_FILE_ID constant for text path in _read_local_tool_result

Replace remaining hardcoded "local" string with the named constant
_LOCAL_TOOL_RESULT_FILE_ID in the text-file return path of
_read_local_tool_result, completing the previous fix that only updated
the binary-file return path.

autogpt_platform/backend/backend/copilot/tools/workspace_files.py

@@ -359,7 +359,7 @@ def _read_local_tool_result(
         )
 
     return WorkspaceFileContentResponse(
-        file_id="local",
+        file_id=_LOCAL_TOOL_RESULT_FILE_ID,
         name=os.path.basename(path),
         path=path,
         mime_type=mimetypes.guess_type(path)[0] or "text/plain",