refactor(agents): move filesystem_backends to app/agents/shared (slice 5d)

Completes slice 5. filesystem_backends was deferred from 3b because it depends
on middleware.{kb_postgres_backend,multi_root_local_folder_backend}; those moved
to shared in 5c, so it now relocates cleanly. Flip the 2 non-frozen importers
(multi-agent factory + test); a re-export shim remains for the frozen
chat_deepagent (build_backend_resolver).

surfsense_backend/app/agents/multi_agent_chat/main_agent/runtime/factory.py

@@ -23,7 +23,7 @@ from app.agents.new_chat.connector_searchable_types import (
     map_connectors_to_searchable_types,
 )
 from app.agents.shared.feature_flags import AgentFeatureFlags, get_flags
-from app.agents.new_chat.filesystem_backends import build_backend_resolver
+from app.agents.shared.filesystem_backends import build_backend_resolver
 from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 from app.agents.shared.llm_config import AgentConfig
 from app.agents.shared.prompt_caching import apply_litellm_prompt_caching

surfsense_backend/app/agents/new_chat/filesystem_backends.py

@@ -1,63 +1,9 @@
-"""Filesystem backend resolver for cloud and desktop-local modes."""
+"""Backward-compatible shim.
 
-from __future__ import annotations
+Moved to ``app.agents.shared.filesystem_backends``. Re-exported here for the
+frozen single-agent stack (``chat_deepagent``) until that stack is retired.
+"""
 
-from collections.abc import Callable
+from app.agents.shared.filesystem_backends import build_backend_resolver
-from functools import lru_cache
 
-from deepagents.backends.protocol import BackendProtocol
+__all__ = ["build_backend_resolver"]
-from deepagents.backends.state import StateBackend
-from langgraph.prebuilt.tool_node import ToolRuntime
-
-from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
-from app.agents.shared.middleware.kb_postgres_backend import KBPostgresBackend
-from app.agents.shared.middleware.multi_root_local_folder_backend import (
-    MultiRootLocalFolderBackend,
-)
-
-
-@lru_cache(maxsize=64)
-def _cached_multi_root_backend(
-    mounts: tuple[tuple[str, str], ...],
-) -> MultiRootLocalFolderBackend:
-    return MultiRootLocalFolderBackend(mounts)
-
-
-def build_backend_resolver(
-    selection: FilesystemSelection,
-    *,
-    search_space_id: int | None = None,
-) -> Callable[[ToolRuntime], BackendProtocol]:
-    """Create deepagents backend resolver for the selected filesystem mode.
-
-    In cloud mode the resolver returns a fresh :class:`KBPostgresBackend`
-    bound to the current ``runtime`` so the backend can read staging state
-    (``staged_dirs``, ``pending_moves``, ``files`` cache, ``kb_anon_doc``,
-    ``kb_matched_chunk_ids``) for each tool call. When no ``search_space_id``
-    is provided, the resolver falls back to :class:`StateBackend` (used by
-    sub-agents and tests that don't need DB-backed reads).
-
-    Desktop-local mode unchanged.
-    """
-
-    if selection.mode == FilesystemMode.DESKTOP_LOCAL_FOLDER and selection.local_mounts:
-
-        def _resolve_local(_runtime: ToolRuntime) -> MultiRootLocalFolderBackend:
-            mounts = tuple(
-                (entry.mount_id, entry.root_path) for entry in selection.local_mounts
-            )
-            return _cached_multi_root_backend(mounts)
-
-        return _resolve_local
-
-    if search_space_id is not None:
-
-        def _resolve_kb(runtime: ToolRuntime) -> BackendProtocol:
-            return KBPostgresBackend(search_space_id, runtime)
-
-        return _resolve_kb
-
-    def _resolve_state(runtime: ToolRuntime) -> StateBackend:
-        return StateBackend(runtime)
-
-    return _resolve_state

surfsense_backend/app/agents/shared/filesystem_backends.py

@@ -0,0 +1,63 @@
+"""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.protocol import BackendProtocol
+from deepagents.backends.state import StateBackend
+from langgraph.prebuilt.tool_node import ToolRuntime
+
+from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
+from app.agents.shared.middleware.kb_postgres_backend import KBPostgresBackend
+from app.agents.shared.middleware.multi_root_local_folder_backend import (
+    MultiRootLocalFolderBackend,
+)
+
+
+@lru_cache(maxsize=64)
+def _cached_multi_root_backend(
+    mounts: tuple[tuple[str, str], ...],
+) -> MultiRootLocalFolderBackend:
+    return MultiRootLocalFolderBackend(mounts)
+
+
+def build_backend_resolver(
+    selection: FilesystemSelection,
+    *,
+    search_space_id: int | None = None,
+) -> Callable[[ToolRuntime], BackendProtocol]:
+    """Create deepagents backend resolver for the selected filesystem mode.
+
+    In cloud mode the resolver returns a fresh :class:`KBPostgresBackend`
+    bound to the current ``runtime`` so the backend can read staging state
+    (``staged_dirs``, ``pending_moves``, ``files`` cache, ``kb_anon_doc``,
+    ``kb_matched_chunk_ids``) for each tool call. When no ``search_space_id``
+    is provided, the resolver falls back to :class:`StateBackend` (used by
+    sub-agents and tests that don't need DB-backed reads).
+
+    Desktop-local mode unchanged.
+    """
+
+    if selection.mode == FilesystemMode.DESKTOP_LOCAL_FOLDER and selection.local_mounts:
+
+        def _resolve_local(_runtime: ToolRuntime) -> MultiRootLocalFolderBackend:
+            mounts = tuple(
+                (entry.mount_id, entry.root_path) for entry in selection.local_mounts
+            )
+            return _cached_multi_root_backend(mounts)
+
+        return _resolve_local
+
+    if search_space_id is not None:
+
+        def _resolve_kb(runtime: ToolRuntime) -> BackendProtocol:
+            return KBPostgresBackend(search_space_id, runtime)
+
+        return _resolve_kb
+
+    def _resolve_state(runtime: ToolRuntime) -> StateBackend:
+        return StateBackend(runtime)
+
+    return _resolve_state

surfsense_backend/tests/unit/middleware/test_filesystem_backends.py

@@ -2,7 +2,7 @@ from pathlib import Path
 
 import pytest
 
-from app.agents.new_chat.filesystem_backends import build_backend_resolver
+from app.agents.shared.filesystem_backends import build_backend_resolver
 from app.agents.shared.filesystem_selection import (
     ClientPlatform,
     FilesystemMode,