coder: telemetry: update task-related telemetry following refactor #981

In coderd/telemetry, we currently track track usage via

• workspace_builds.has_ai_task
• template_versions.has_ai_task

We are removing workspace_builds.has_ai_task in #973
This should be replaced supplemented by adding a top-level telemetry.Task struct to telemetry snapshots.

Existing use of template_versions.has_ai_task can remain.

Steps:

• Update coderd/telemetry package to include database.Task entries feat(coderd/telemetry): add telemetry for database Tasks coder#20279
• Update coder-telemetry-server to handle the new snapshot fields https://github.com/coder/coder-telemetry-server/pull/30
• Perform BigQuery migration EDIT: this step is now performed automatically