[recipe] feat: support reward_loop for recipe/fully_async_policy

[sl-1314]

What does this PR do?

This PR mainly supports reward_loop for recipe/fully_async_policy.
The main changes are as follows:

• refactor recipe/fully_async_policy to support reward_loop
• fix recipe/fully_async_policy final validation bug
• extract _agent_loop_postprocess: Move the padding/mask/position-id post-processing logic out of _run_agent_loop into a dedicated function async def _agent_loop_postprocess(...).

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[gemini-code-assist]

Code Review

This pull request introduces support for reward_loop in the fully asynchronous policy, involving a significant refactoring of the rollout process. The changes simplify the data flow by removing the intermediate result_queue and integrate the consumer logic directly into the processor. The fix for the final validation logic also appears more robust. My review focuses on improving exception handling for better debuggability and addressing code duplication to enhance maintainability.