[RLlib] Fix wrong env being passed into on_episode_end callback on MultiAgentEnvRunner when sampling whole episodes.

[sven1977]

Fix wrong env being passed into on_episode_end callback on MultiAgentEnvRunner when sampling whole episodes.

• Enhance test cases to capture proper callbacks arguments (env, env_runner, metrics_logger).

Why are these changes needed?

Related issue number

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• Testing Strategy
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[simonsays1980]

LGTM.