Fix project imports not working with serialized data

The change in e404c311 broke models that relied on single-type inheritance:

## Before:
CommitStatus.new(type: 'Ci::Build').class
=> Ci::Build

## After the change:
CommitStatus.new.tap{|c| c.assign_attributes(type: 'Ci::Build')}.class
=> CommitStatus

As a result, any serialized options would not be properly saved to the database, leading to errors expecting a Hash vs. a String, for example.

This affected all pipelines that used shared runners because the build would error out due to this serialization error.

Closes #34860 (closed)