Create E2E test to generate an MR summary

Test the Summarize Code Changes feature to ensure feature is working correctly from an end to end point of view. Test will need to be executed in a live environment to integrate with a AI provider.

Related feedback issue: #443236 (closed) Related AI gateway inteway integration issue #459868 (closed)

TODO: need to confirm whether the current form of the feature will be the GA feature

Note: to enable feature in GDK, group needs to be ultimate: