feat: record the model as its own field in the audit log

Feature request

Record the model as a dedicated field on each audit log entry.

Today the model name only shows up because the raw request body is copied into the entry, and that copy is cut off at MAX_BODY_LOG_SIZE (2048 bytes by default). As soon as the body is bigger than the limit and model sits after messages in the JSON (which is what the official OpenAI SDKs send), the model is missing from the entry. A large system prompt alone is enough.

Raising MAX_BODY_LOG_SIZE only moves the limit: every entry then carries the full prompt text, which is what most operators want to avoid, and the next longer prompt breaks it again.

Wanted

The model recorded as its own field, independent of body size, key ordering and MAX_BODY_LOG_SIZE.

---

Requested by @gaby on Discord