FEAT Migrate consumers to TargetConfiguration capability checks

[hannahwestra25]

Summary

Replaces scattered isinstance(target, PromptChatTarget) checks with explicit, declarative capability requirements validated against TargetConfiguration. Consumers now care about capabilities, not concrete target types.

The bigger picture

Next step in the migration from type-based gating to the TargetConfiguration / TargetCapabilities / CapabilityHandlingPolicy model. With this change, scorers, converters, and attacks can accept any PromptTarget whose configuration satisfies their declared requirements — natively or via an ADAPT normalization pipeline — opening non-chat targets to flows previously locked to chat-shaped APIs.

What changed

• set_system_prompt moved from PromptChatTarget onto PromptTarget (capability gates behavior, not subclass identity).
• TargetRequirements gains:
  • required — satisfied by native support or ADAPT policy.
  • native_required — must be natively supported; ADAPT is rejected.
  • validate(*, target) — enforces both tiers, called automatically by base classes.
• CHAT_CONSUMER_REQUIREMENTS introduced as the shared constant for scorers/converters.
• Scorers and converters declare TARGET_REQUIREMENTS = CHAT_CONSUMER_REQUIREMENTS and validate in __init__; chat_target / converter_target params typed as PromptTarget.
• AttackStrategy exposes TARGET_REQUIREMENTS and validates the objective_target in its __init__. CrescendoAttack, ChunkedRequestAttack, and MultiPromptSendingAttack declare their MULTI_TURN need as native_required — adaptation would silently collapse their multi-turn semantics, so it's rejected at construction time.
• ConversationManager.set_system_prompt accepts PromptTarget and calls ensure_can_handle(SYSTEM_PROMPT). Prepended-conversation routing switched from isinstance(PromptChatTarget) to a capability check.
• Tests cover validate() across native support, ADAPT, RAISE, and empty-required cases.

Out of scope

No behavior change for targets that already satisfied the old PromptChatTarget contract. Custom adaptation still plugs in through CapabilityHandlingPolicy / normalizer overrides.

Tests and Documentation

Added/Updated tests

[jsong468]

Should we be using TargetRequirements for scenarios like we do with attacks?

[jsong468]

This is because of backtracking right? Maybe add the reason in the docstring above (NIT)

[jsong468]

Also, if I'm understanding correctly, doesn't this mean _GPT_4O and many other targets won't support crescendo? based on the known capability profiles section of target_capabilities.py? maybe just that section needs to change to align with OpenAIChatTargets supporting editable history.