fix: do not remove last identity provider

[aditya-mitra]

Summary

• Do not show the OAuth Identity Provider for removal if it is the only one present.
• Also, added the check in server-core along with corresponding tests.
• Refactored identity-provider service tests to use userId instead of providers array. (assert should not be done inside a loop)

References

Closes #7575

Checklist

• If this PR is still a WIP, convert to a draft
• When this PR is ready, mark it as "Ready for review"
• ensure all checks pass
• Changes have been manually QA'd
• Changes reviewed by at least 2 approved reviewer

QA Steps

List any additional steps required to QA the changes of this PR, as well as any supplemental images or videos.

[HexaField]

This looks good, we should also ensure this in the backend service.