core(emulation): throw on host environment / settings mismatches

An extra check I wanted to add on top of #11779

It was described back in #10910

> the TestedAsMobileDevice logic correctly handles real-mobile-device, but doesn't handle external UA emulation. We could detect there's external UA emulation being applied and throw if we don't see configuration that matches. Or with the proposal implemented, we may just log a warning to let them know we see i

We can only check this sort of mismatch after we've gathered the host UA, so it can't be done earlier.