refactor(agents): move MAC graph-state schema into multi_agent_chat/shared/state/

filesystem_state.py (the multi-agent graph state) and state_reducers.py
(its merge reducers) are consumed only by multi_agent_chat (filesystem
tools/middleware, kb projection, and the MAC-only shared middleware) plus
two unit tests -- no external app code. Relocate them into a dedicated
multi_agent_chat/shared/state/ package (filesystem_state.py + reducers.py)
and repoint every importer.

No behavior change; import-all + the full unit/middleware + unit/agents
suites (1066 tests) stay green.

surfsense_backend/tests/unit/agents/new_chat/test_state_reducers.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 import pytest
 
-from app.agents.shared.state_reducers import (
+from app.agents.multi_agent_chat.shared.state.reducers import (
     _CLEAR,
     _add_unique_reducer,
     _dict_merge_with_tombstones_reducer,

surfsense_backend/tests/unit/middleware/test_b_filesystem_rm_rmdir_cloud.py

@@ -22,10 +22,10 @@ import pytest
 from app.agents.multi_agent_chat.shared.middleware.filesystem import (
     build_filesystem_mw,
 )
+from app.agents.multi_agent_chat.shared.state.reducers import _CLEAR
 from app.agents.shared.filesystem_backends import build_backend_resolver
 from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 from app.agents.shared.middleware.kb_postgres_backend import KBPostgresBackend
-from app.agents.shared.state_reducers import _CLEAR
 
 pytestmark = pytest.mark.unit