Site Migration: Confirm `migration_source_site_domain` blog option is being saved and persisted to Atomic

[sixhours]

While working on #101697, I noticed that we seem to be losing the migration_source_site_domain blog option upon transfer to Atomic. We'd added some code to persist it in rWP314275-code, but I don't think it's working or something has changed. This needs more investigation because we rely on that option to direct the user during the migration process.

[sixhours]

Confirmed we added the code to persist the option here: 172545-ghe-Automattic/wpcom

And confirmed we're setting it in the API endpoint here: fbhepr%2Skers%2Sjcpbz%2Sjc%2Qpbagrag%2Serfg%2Qncv%2Qcyhtvaf%2Sraqcbvagf%2Sfvgrf%2Qngbzvp%2Qgenafsref.cuc%3Se%3Qs1n83831%23410-og

But when I check the option after the transfer, it's empty. I suspect this is a timing issue where we're updating the option while the site is being transferred and it doesn't stick.

@a8caustin Should we be setting the option in a different way, maybe as transfer meta?

[sixhours]

Addressing this in 179225-ghe-Automattic/wpcom

[sixhours]

This appears to be working now that the option has been added in the correct place!