Mob 50542 isolated session id fix

[alejandroaires]

Summary

• Stop using Mcp-Session-Id / FastMCP ctx.session_id as the Storage partition key. Hosted HTTP reuses one MCP session across chats, so dataframes and tasks leaked between conversations.
• Partition key is now {user_id}/{session_scope_id}:
  1. args.session_scope_id from a previous successful tool result in the same chat
  2. otherwise a minted id for that tool call
• Successful tool results include session_scope_id. Agents must reuse it on later calls in the same chat and must not copy it from another chat.

Flow

• New HTTP chat: first tool mints a session_scope_id; later tools in that chat reuse it and see the same dataframes/tasks
• Second chat: do not pass the first chat’s session_scope_id; lists start empty
• Confirm dataframes_list / tasks_list do not show state from another chat on the same hosted MCP session