Fix env checks for async environments

[atyshka]

What this does

Async environments have been broken ever since OpenPi added new checks to the environments in #921. This refactors the checks where possible, but I could not find a way to implement the type check for async envs, so this check is skipped with a warning.

How it was tested

I trained smolvla with the aloha sim environment.

How to checkout & try? (for the reviewer)

lerobot-train \
    --policy.type=act \
    --dataset.repo_id=lerobot/aloha_sim_insertion_human \
    --env.type=aloha \
    --output_dir=outputs/train/act_aloha_insertion
    --env.use_async_envs=true

[Copilot]

Pull Request Overview

This PR fixes environment checks for async environments by refactoring the way environment attributes are accessed and validated. The changes address issues where async environments were broken due to new environment checks that didn't work with the async vector environment pattern.

Key changes:

• Replaces direct attribute access on env.envs[0] with safer env.call() method calls
• Adds exception handling for environment method calls to gracefully handle missing attributes
• Skips type checking for AsyncVectorEnv with a warning since it cannot be reliably implemented