fix(order-stream): make WebSocket cleanup generation-aware

[grandpig]

What

Make WebSocket connection cleanup identity-aware.

Fixes #2066

Why

Connection cleanup currently removes entries by address only. When a client reconnects with the same address, cleanup from the previous connection can remove the replacement connection. Broadcast cleanup has the same race.

Changes

• Add a unique connection ID/generation.
• Remove connections only when the stored ID matches the cleaning task.
• Make broadcast cleanup generation-aware.
• Ensure the replaced WebSocket task is explicitly terminated.
• Add deterministic regression tests.

Testing

• cargo test -p order-stream

[grandpig]

@capossele Just following up on this in case it got buried. This fixes the reconnect cleanup race so a stale WebSocket task can't remove its replacement connection, including the same race in broadcast cleanup. Deterministic regression tests are included. Thanks!