Add agent_platform_session stopped and dropped events

What does this MR do and why?

• This MR adds agent_platform_session_stopped and agent_platform_session_dropped events for tracking the stop and drop for a platform session
• Discussion: https://gitlab.com/gitlab-org/gitlab/-/issues/570564#note_2781702567
• Previous MR: !206243 (merged)

How to set up and validate locally

• Create a new agent platform session by running an agent platform session in IDE and stop / drop the session
• Go to postgres gdk psql
• Execute select * from ai_usage_events order by id desc limit 1
• You should see an event record with filled user_id, namespace_id, and extras including flow_type, project_id, session_id, environment

MR acceptance checklist

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Related to #570564