Upsert provider access tokens instead of Create and Delete

[jhrozek]

Summary

For OAuth flow provider enrollments we used to delete and recreate the
provider access token in order to be able to list the new token based on
CreatedAt. For the case where we pass a PAT, we would just error out
in case the token was already created.

Let's just upsert in both cases and instead of CreatedAt let's look at
UpdatedAt.

This will be useful for tests.

Fixes: #2411

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

Manual testing with both PAT enrollment and OAuth flow.

Review Checklist:

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

[coveralls]

coverage: 39.333% (+0.02%) from 39.316%
when pulling 574eb3a on jhrozek:upsert_provider_token
into e68fcce on stacklok:main.