CM-72541: Report the model on every AI guardrails hook event

[Altruistus]

TL;DR

A user opens a chat on one model, switches to another mid-conversation, and the platform keeps attributing every later prompt, file read and MCP call to the model they started on. Hook events now carry the model they ran under, so the platform can attribute each event to the model that actually served it.

Why

The model reached the backend in exactly one place: create_conversation, called by the session-start hook. create_event never carried one, so the model of the conversation's first turn stood in for every event in it.

No IDE starts a new conversation when the user picks a different model. Cursor keeps the same conversation_id when the composer's model changes and fires no sessionStart, so nothing ever corrected the record. Every hook payload already carries the model configured at that moment - Cursor.parse_hook_payload has been reading it into AIHookPayload.model all along - so the CLI had the right answer in hand on every hook and dropped it on the floor.

Reviewer notes

• One field on the event body. Attribution moves from the conversation to the event, which is the level the switch actually happens at; no other CLI behaviour changes.
• model is sent even when it is None (a Claude Code transcript with no assistant turn yet, for one), consistent with the other optional fields on this body.
• The conversation record is untouched. It keeps reporting the model the session opened with; the backend is what joins events to models from here.

Reproduced against the ticket's flow in Cursor - a conversation opened on grok-4.6 and switched to gpt-5.6-sol.

CM-72541

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