Add SubQuestionSources, SubQuestionResult, GeneratingSubquestionEvent Pydantic models for the new per-sub-question response format. Add chunks_retrieved_per_subq_count and chunks_filtered_per_subq_count optional fields to QueryHistoryRecord and QueryHistoryDetail for per-sub-question chunk count tracking.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>