[Backport] fix: only auto-submit SSO registration form when provider returns an email

[ayesha-waris]

Backport of #38860 to the release/verawood branch.

Description

When a third-party auth provider has "skip registration form" enabled, the authn MFE context set autoSubmitRegForm=True unconditionally. Providers such as Facebook (email permission denied) or Microsoft Entra ID can complete authentication without returning an email claim; auto-submitting the registration form in that case silently fails client-side validation and leaves the learner stuck on a partially-filled form with no explanation.

This change gates the auto-submit on the provider actually returning an email so the learner falls back to the normal registration form and can fill in what is missing.

Backport details

• Original PR: fix: only auto-submit SSO registration form when provider returns an email #38860 (merged to master)
• Cherry-picked cleanly from commit 773ead2b7cb6d0d5bb14ee450ac3876eadeb5fca
• Fixes: SSO flow does not skip registration form when "Skip registration form" is enabled #38780

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