fix(server): scope disconnect cleanup to current connection

[ZingerLittleBee]

Summary

• move temporary grant cleanup into the shared current-connection removal path
• prevent a superseded Agent WebSocket from clearing grants reported by the active replacement connection
• add end-to-end regression coverage through the Agent WebSocket and REST server DTO

Why

The superseded socket cleanup previously cleared temporary grants unconditionally, even when connection fencing correctly rejected removal of the stale connection ID. This could erase runtime state that the replacement connection had just reported.

Testing

• cargo test -p serverbee-server --lib service::agent_manager::tests::
• cargo test -p serverbee-server --test ws_agent_dispatch2
• cargo clippy -p serverbee-server --test ws_agent_dispatch2 --all-features
• mutation check: restoring the old unconditional cleanup makes the new reconnect regression test fail