Persist event source to source_type on duo workflow creation

What does this MR do and why?

Reconciles the source event property with the source_type database column on duo_workflows_workflows.

Previously, the source param (e.g. merge_request_code_conflict, fix_pipeline) was only forwarded to the agent_platform_session_created internal analytics event and was never persisted to the database. The source_type column existed but only held slack and mcp.

This MR:

1. Extends the source_type enum in Ai::DuoWorkflows::Workflow to include all internal GitLab entry points from WORKFLOW_ACTIONS_SOURCE.
2. Persists the source param into source_type at workflow creation time via a new source_type_attributes private method in CreateWorkflowService. Unknown or blank source values are silently ignored, preserving backward compatibility.

The source_type column already exists in the database (added in milestone 19.4 by AddSourceTypeAndSourceLinkToDuoWorkflowsWorkflows), so no migration is needed.

References

• #617810 (closed) (this issue)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/605256 (event source tracking)
• #603759 (closed) (source_type + source_link DB columns)
• !247849 (merged) (original source_type migration)

How to set up and validate locally

1. Run a fix pipeline flow on a failed pipeline
2. In the rails console, confirm source_type was saved:

pry(main)> Ai::DuoWorkflows::Workflow.last.source_type
...
=> "fix_pipeline"

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