FEAT add TargetRequirements

[hannahwestra25]

Description

Adds TargetRequirements, the consumer-side counterpart to TargetConfiguration. Useful for consumers (attacks, converters, scorers) to validate their requirements against a target's configuration at construction time. TargetRequirements is the mechanism for that — a frozen, declarative description of what capabilities a consumer needs, validated in a single call.

for example, a CrescendoAttack would use requirements :

 class CrescendoAttack:
    REQUIREMENTS = TargetRequirements(
        required_capabilities=frozenset({
            CapabilityName.MULTI_TURN,
            CapabilityName.SYSTEM_PROMPT,
        })
    )


    def __init__(self, *, objective_target: PromptTarget, ...) -> None:
        # Validates all requirements in one call — fails fast with all violations
        self.REQUIREMENTS.validate(configuration=objective_target.configuration)

If the target natively supports the capabilities, validation passes. If not, the TargetConfiguration's CapabilityHandlingPolicy is consulted — if the policy is ADAPT, normalization will handle it; if RAISE (or no policy exists), validate() collects all violations and raises a single ValueError eg:
ValueError: Target does not satisfy 2 required capability(ies):

• Target does not support 'supports_multi_turn' and the handling policy is RAISE.
• Target does not support 'supports_system_prompt' and no handling policy exists for it.

Tests and Documentation

[ValbuenaVC]

Two nits, looks good!

[ValbuenaVC]

nit: isn't this guaranteed by Python itself? Or do you want to just make sure that the attribute is frozen rather than that freezing works

[ValbuenaVC]

Nit: I'm not sure I understand the point of this dataclass. What does this do that a validate method attached to TargetConfiguration or TargetCapabilities doesn't? Attaching TargetCapabilities to a consumer object is weird but this class seems like it could be reduced to a private attribute for consumers (_REQUIRED_CAPABILITIES: frozenset) while the validation method is moved elsewhere. Non-blocking comment since this works but worth documenting