refactor(ppo): remove performance metrics config compatibility after BaseModel migration

## Context

PR #2518 migrates the GRPO config from TypedDict to Pydantic BaseModel. `print_performance_metrics` is shared by GRPO and PPO, so it temporarily supports both attribute-style GRPO config and dict-style PPO config.

Once PPO config is migrated to BaseModel, remove this compatibility path and use direct typed attribute access throughout.

## Cleanup scope

- Replace the GRPO/PPO `getattr` algorithm detection in `print_performance_metrics` with typed config access.
- Remove the `isinstance(algo_config, dict)` branch for prompt and generation counts.
- Remove the remaining temporary top-level GRPO guard around async metrics if the shared API is split or typed accordingly.
- Update the corresponding unit tests.

Tracked from the review discussion on #2518: https://github.com/NVIDIA-NeMo/RL/pull/2518#discussion_r3692119732