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Benjamin Carlsson e4568a3538 Reject TUI events from a different backend (#55304)
## Why

Events from another backend must not mutate the current backend's thread or connection state, even when native thread IDs collide.

## What changed

Attach the session's `BackendId` to incoming app-server events and check it before dispatching to thread caches or UI handlers. Apply the same check when waiting for a thread revert notification.

## Testing

Add regression tests covering ignored foreign notifications, approval requests, lag and disconnect events, accepted events from the current backend, and preservation of backend identity on transport events.

GitOrigin-RevId: 2577ab78e0592726f5f05e3426772899e6cb7c7e
2026-10-19 15:20:42 +00:00
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