Validate goal_integrity requires expected_goal

[mertsatilmaz]

Goal

Update scenario validation so goal_integrity assertions require a non-empty expected_goal.

Why this matters

goal_integrity now depends on assertion-level configuration:

assertions:
  - type: goal_integrity
    expected_goal: summarize_document

[Flames4fun]

Hi! I’d like to help with this issue :))

It looks like a good first contribution to understand the assertion/validation flow in the project. Could you please assign it to me?

Thanks!