Update and quarantine pull mirroring test

What does this MR do and why?

• Updates the test to verify mirroring via the API before checking the UI.
• Includes the error in the test failing message when mirroring fails.
• Adds the deploy key via the API.
• Quarantines the test on Staging because it fails intermittently and the cause seems to be the test environment.

See #259481 (closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.