Add start_agent_platform_session event to GitLab instance

Problem

We are currently not tracking the start of a GitLab Duo Agent platform (formerly Duo Workflow) session from within the instance. Not getting this data from individual instances means we cannot show the data back to the user as part of AI tracking. The main important part is whether a specific session got started.

Desired Outcome

The start of a session is tracked together with the most important information.

Implementation Plan

1. Instrument a new internal event (quick start guide)

   1. name: start_agent_platform_session
   2. label: [flow_type] (e.g. chat, software_development etc.)
   3. value: [session_id] (still called workflow_id in most places)

2. Make sure project and user information is passed as described in the docs.

3. Add the event to AI tracking as described in !197129 (diffs).

4. Additionally change the label for the existing cleanup_stuck_agent_platform_session to also include the flow_type