FEAT Support baseline-only execution in Scenario

[slister1001]

Summary

This PR enables scenarios to be initialized with an empty strategies list when include_baseline=True. Previously, empty strategies would raise a ValueError even when baseline was requested.

Problem

When using RedTeamAgent (or FoundryScenario) with include_baseline=True and scenario_strategies=[], the scenario would fail with:

ValueError: No valid FoundryStrategy strategies provided.

This prevented users from running baseline-only attacks, which is useful for:

• Establishing baseline metrics before applying attack techniques
• Quick sanity checks of target behavior
• Comparing raw objective success rates against attack-modified prompts

Changes

1. scenario_strategy.py: Added allow_empty parameter to prepare_scenario_strategies(). When True and an empty sequence is explicitly provided, returns an empty list instead of raising ValueError.

2. Tests: Added unit tests covering baseline-only execution scenarios & updated tests to use dataset_config rather than objectives

Usage Example

scenario = RedTeamAgent(
    include_baseline=True,
    attack_scoring_config=scoring_config,
)

await scenario.initialize_async(
    objective_target=target,
    scenario_strategies=[],  # Empty - baseline only
    dataset_config=dataset_config,
)

result = await scenario.run_async()
# Result contains only baseline attack results

Test Results

All tests passing