fix(metrics): include dropped workflow episodes in denominators

[rajatbeladiya]

Issue #382 reports that workflow episodes that terminate before producing any steps (e.g. max_prompt_length_exceeded) are silently dropped (repeat_counts=0) and not counted in metrics, inflating pass@k and hiding termination reasons.

This PR:

• Tracks dropped episodes in AgentWorkflowEngine.transform_results_for_verl and returns them in meta_info.dropped_episodes.
• Updates AgentWorkflowTrainer to:
  • include dropped termination reasons in termination_counts
  • use num_tasks as denominator for termination metrics
  • account for dropped episodes in validation pass@k by treating them as incorrect.
• Adds a regression test to ensure dropped episodes are surfaced in meta_info.

Fixes #382