refactor(agents): sink sandbox.py into filesystem subsystem

shared/sandbox.py was used only by the filesystem middleware/tools (and the
boundary) -- never by main_agent or subagents as shared code. Move it next to
its only agent-side consumer:

  multi_agent_chat/shared/sandbox.py
  -> multi_agent_chat/shared/middleware/filesystem/sandbox.py

surfsense_backend/app/agents/chat/multi_agent_chat/shared/middleware/filesystem/middleware/middleware.py

@@ -8,7 +8,9 @@ from deepagents import FilesystemMiddleware
 from langchain_core.tools import BaseTool
 
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import FilesystemMode
-from app.agents.chat.multi_agent_chat.shared.sandbox import is_sandbox_enabled
+from app.agents.chat.multi_agent_chat.shared.middleware.filesystem.sandbox import (
+    is_sandbox_enabled,
+)
 from app.agents.chat.multi_agent_chat.shared.state.filesystem_state import (
     SurfSenseFilesystemState,
 )

surfsense_backend/app/agents/chat/multi_agent_chat/shared/middleware/filesystem/tools/execute_code/helpers.py

@@ -14,7 +14,7 @@ from typing import TYPE_CHECKING
 from daytona.common.errors import DaytonaError
 from langchain.tools import ToolRuntime
 
-from app.agents.chat.multi_agent_chat.shared.sandbox import (
+from app.agents.chat.multi_agent_chat.shared.middleware.filesystem.sandbox import (
     _evict_sandbox_cache,
     delete_sandbox,
     get_or_create_sandbox,