fix room_selector example which always run the first task

[TerryChan]

In the room_selector example, the runner incorrectly executes the first task (tasks[0]) on every step instead of advancing to the next one. As a result, the example produces inconsistent outputs: the expected_choice and final result are mismatched, but the model is still assigned a high reward.

[ultmaster]

/ci

[github-actions[bot]]

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