Track request_channel on import lifecycle events

What does this MR do and why?

Adds request_channel (api / ui / congregate) as an additional property on the project and group import start events, so imports can be broken down by how they were started. Congregate is a Professional Services migration tool that calls the API; it's identified by its GitLabApiClient User-Agent.

request_channel is only sent on start_project_import and start_group_import. The channel can't change partway through an import, and for project imports the value is held in a Redis cache that expires after 24 hours, so a long-running import could lose it by the time its later events fire.

Implementation

Two capture paths, both write to Redis via Gitlab::Cache::Import::Caching — no new columns, matching the pattern used by this MR.

• Direct Transfer / Offline Transfer: captured in Import::BulkImports::EphemeralData at BulkImports::CreateService time; read from BulkImports::Entity when the start events fire.
• Standard importers inheriting Import::BaseService (GitHub, Gitea, Bitbucket, Bitbucket Server, Fogbugz): the calling controller or API endpoint sets request_channel= on the service, which stashes it in the request store via Gitlab::Import::RequestChannel.stash; ProjectImportState reads it with .stashed in an after_create callback and writes it to Redis; it's read back when start_project_import fires.
• GroupImportState doesn't capture request_channel yet; file-based group import is covered by the follow-up.

Detection helper: Gitlab::Import::RequestChannel.detect(request) returns :congregate for the GitLabApiClient UA, :api otherwise. Controllers pass :ui explicitly.

Deliberately not added to *_gitlab_migration_entity events per the design in #617884.

Scope

• Part of #617884.
• Closes #627877 (closed).
• Follow-up #630178 (closed) covers the remaining importers that don't inherit Import::BaseService (Bitbucket UI controller, Manifest, Git URL, file-based project/group).

How to validate locally

1. Enable Snowplow Micro:

   gdk config set snowplow_micro.enabled true
   gdk reconfigure && gdk restart

2. Watch http://gdk.test:9091/micro/good.

3. Start imports through each channel:

   • UI: any importer via New Project → Import.
   • API: curl -H "PRIVATE-TOKEN: <pat>" -X POST http://gdk.test:3000/api/v4/import/github ....
   • Congregate: same, adding -H "User-Agent: GitLabApiClient".

4. Confirm start_project_import / start_group_import carry request_channel: 'ui' / 'api' / 'congregate', and that the finish/fail/cancel/timeout events don't.

MR acceptance checklist

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Related to #627877 (closed)