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/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,