feat(filesystem): add local folder backend and verification coverage

surfsense_backend/tests/unit/middleware/test_filesystem_backends.py

@@ -0,0 +1,37 @@
+from pathlib import Path
+
+import pytest
+
+from app.agents.new_chat.filesystem_backends import build_backend_resolver
+from app.agents.new_chat.filesystem_selection import (
+    ClientPlatform,
+    FilesystemMode,
+    FilesystemSelection,
+)
+from app.agents.new_chat.middleware.local_folder_backend import LocalFolderBackend
+
+pytestmark = pytest.mark.unit
+
+
+class _RuntimeStub:
+    state = {"files": {}}
+
+
+def test_backend_resolver_returns_local_backend_for_local_mode(tmp_path: Path):
+    selection = FilesystemSelection(
+        mode=FilesystemMode.DESKTOP_LOCAL_FOLDER,
+        client_platform=ClientPlatform.DESKTOP,
+        local_root_path=str(tmp_path),
+    )
+    resolver = build_backend_resolver(selection)
+
+    backend = resolver(_RuntimeStub())
+    assert isinstance(backend, LocalFolderBackend)
+
+
+def test_backend_resolver_uses_cloud_mode_by_default():
+    resolver = build_backend_resolver(FilesystemSelection())
+    backend = resolver(_RuntimeStub())
+    # StateBackend class name check keeps this test decoupled
+    # from internal deepagents runtime class identity.
+    assert backend.__class__.__name__ == "StateBackend"