upgrade Azure OpenAI from GPT-4o to ≥5.1 (Foundry retirement)

# Context

Azure AI Foundry is retiring our current GPT-4o deployment; we need a supported model (≥5.1).

# Outcome

All LLM calls use a ≥5.1 model in Foundry; configs/docs/env updated; behavior validated.

# Acceptance criteria

- Default (or required) deployment targets a ≥5.1 model; no remaining dependency on retired GPT-4o for production paths.
- Smoke or targeted tests pass; prompt/JSON behavior acceptable for existing flows.