fix: fix flattening in complete

[AayushSabharwal]

The old implementation broke Catalyst, but it wasn't detected due to differing BifurcationKit compats preventing tests from running.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

Extension test failing? Seems unrelated though.

[AayushSabharwal]

Huh, weird, it only fails on lts.

[ChrisRackauckas]

Must be an alternative solution or something, just choosing the wrong root?

[AayushSabharwal]

Nope, it's a bug. x = 2 is an invalid root because it occurs in the denominator, but I forgot to add an abs in the check so it accepts 2 - 2e-8 but rejects 2 + 2e-8.