feat(new-chat): add filesystem backend interfaces and selection helpers

surfsense_backend/app/agents/new_chat/filesystem_backends.py

@@ -0,0 +1,38 @@
+"""Filesystem backend resolver for cloud and desktop-local modes."""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+from functools import lru_cache
+
+from deepagents.backends.state import StateBackend
+from langgraph.prebuilt.tool_node import ToolRuntime
+
+from app.agents.new_chat.filesystem_selection import FilesystemMode, FilesystemSelection
+from app.agents.new_chat.middleware.local_folder_backend import LocalFolderBackend
+
+
+@lru_cache(maxsize=64)
+def _cached_local_backend(root_path: str) -> LocalFolderBackend:
+    return LocalFolderBackend(root_path)
+
+
+def build_backend_resolver(
+    selection: FilesystemSelection,
+) -> Callable[[ToolRuntime], StateBackend | LocalFolderBackend]:
+    """Create deepagents backend resolver for the selected filesystem mode."""
+
+    if (
+        selection.mode == FilesystemMode.DESKTOP_LOCAL_FOLDER
+        and selection.local_root_path is not None
+    ):
+
+        def _resolve_local(_runtime: ToolRuntime) -> LocalFolderBackend:
+            return _cached_local_backend(selection.local_root_path or "")
+
+        return _resolve_local
+
+    def _resolve_cloud(runtime: ToolRuntime) -> StateBackend:
+        return StateBackend(runtime)
+
+    return _resolve_cloud