Don't pre-create github provider on registration

[psekar]

Summary

On the first user registration, a provider entry is created in the database. Although, the provider cannot be used to register repos due to the lack of credential. As part of this change, the default provider will not be created and user will continue with provider enroll step.

Provide a brief overview of the changes and the issue being addressed.
Explain the rationale and any background necessary for understanding the changes.
List dependencies required by this change, if any.

Fixes #(related issue)
#4213

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.
After the first user login, verify the provider list returns empty. Continue with provider enrollment and repo registration should successfully complete.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.646% (-0.007%) from 53.653%
when pulling a9fb173 on tinytrail:user-registration-donotcreate-provider
into bec2b46 on stacklok:main.

[eleftherias]

Thank you for your contribution @psekar!

note: The failing coverage step is because of one of our flaky tests (we're looking into it), not related to your changes.