Wire `author_source` into `completed_pipeline_execution` and `first_pipeline_succeeded` events

What does this MR do and why?

Part of #596636 — identifying agent-generated pipeline configs to filter Config Quality and Time-to-first-pipeline metrics to AI-guided flows.

Previous MRs in this chain established the foundation:

• Add `author_source` property to CI config inter... (!232453 - merged) added author_source to the commit_change_to_ciconfigfile and create_ci_config_file internal events by reading the Duo-Workflow-Definition git trailer at commit time.
• Add ci_config_author_source column to ci_projec... (!232740 - merged) added the ci_config_generated_by column to ci_project_metrics and a track_ai_generated_config! upsert, which persists the trailer value when an AI agent commits a CI config.

This MR closes the loop by surfacing author_source on the two downstream events that analytics actually uses:

• completed_pipeline_execution — fires on every pipeline completion; author_source lets us segment Config Quality metrics by whether the pipeline was set up by a human or an AI agent.
• first_pipeline_succeeded — fires on Time-to-first-pipeline; author_source lets us compare TTFP for AI-guided onboarding vs. manual onboarding.

Because these events fire from the pipeline execution context, not from a git push, we cannot read the git trailer there directly. Instead we look up the persisted value from ci_project_metrics at event-fire time. When no AI agent trailer was recorded for the project, the value defaults to 'human'.

References

• Issue: Identify agent-generated pipeline configs using... (#596636)
• Persistence foundation: Add ci_config_author_source column to ci_projec... (!232740 - merged)
• CI config commit events: Add `author_source` property to CI config inter... (!232453 - merged)