Query reformulation

[NicolasLMP]

This PR introduces a new ReformulationService that uses a context-aware LLM to rewrite user queries based on conversation history.

Key changes:

- Adds a contextual query reformulation via LLM

Also:

• Removed legacy semantic expansion (5-query logic, static intent detection)
• Unified pipeline: Reformulation → Retrieval → Generation
• Query now consistently used for both vector search and filtering
• Enhanced document filtering with better French support and fallback logic