Create Preliminary Participant Endpoint

Creates a preliminary Lead participant from step 1 (participant details) form data.

Called by the forms app after the participant details step is successfully submitted, before the form is fully completed. This ensures a participant record exists in the database with status="Lead" and status_category="Form in progress" as soon as a user starts filling in their details.

The participant ID is stored in the form_request processing_outcomes so that form_completion_background_task skips re-creation and instead updates the existing record.

Not valid for bank_details_form requests — those are pre-linked to an existing entity and must never create new participants.