TEST: Add unit tests for pyrit/models/ module

[romanlutz]

Summary

Adds unit tests for 10 previously untested model files including seed models.

New Test Files

Source File	Tests	Coverage
chat_message.py	17	ToolCall, ChatMessage (init, serialization, roles), ChatMessagesDataset
conversation_reference.py	10	ConversationType enum, frozen dataclass, hash, equality
conversation_stats.py	5	Defaults, values, frozen immutability
question_answering.py	14	QuestionChoice, QuestionAnsweringEntry, Dataset
scenario_result.py	12	ScenarioIdentifier, ScenarioResult (strategies, objectives, success rate)
strategy_result.py	3	duplicate() deep copy, independence
seeds/seed_attack_group.py	7	Valid init, objective property, validation errors
seeds/seed_dataset.py	18	Init, get_values, random, from_dict, grouping, deprecation
seeds/seed_objective.py	7	Init, data_type, technique errors, Jinja rendering
seeds/seed_prompt.py	14	Init, data_type inference, from_messages

Testing

All 121 tests pass locally.