LG-10050: Prevent MFA setup validation error from persisting

[aduth]

🎫 Ticket

LG-10050

🛠 Summary of changes

Updates validation logic for required MFA selection during account creation to prevent the "You must select an authentication method" from lingering unexpectedly after making a selection.

This includes some general refactoring to validation and error handling logic, to reinforce expectations around the shape and value formats of expected parameters and how (and where) error messages are computed.

The fix itself is using flash.now instead of flash where a redirect is not involved.

📜 Testing Plan

1. Go to http://localhost:3000
2. Click "Create an account"
3. Complete account creation up to MFA selection screen
4. Click "Submit" without selecting an MFA method
5. Observe the validation error "Select an authentication method"
6. Select an MFA method (e.g. phone) and click "Submit"

Before: The error message would persist onto the MFA setup screen.
After: The error message does not persist onto the MFA setup screen.

👀 Screenshots

Before	After

[mitchellhenke]

Hm, this is another one that would have been caught by Rails/ActionControllerFlashBeforeRender

Was mentioned in #8961, but not enabled due to false positives. Maybe we should consider enabling it?

[aduth]

Was mentioned in #8961, but not enabled due to false positives. Maybe we should consider enabling it?

Maybe! I'd be curious to test to see what the false positives look like.

Edit: For posterity, see #8986