Reference code allows invalid time strings with fractional seconds but without minutes

[ptomato]

The reference polyfill allows this:

> Temporal.PlainTime.from('05:07.123')
Temporal.PlainTime <05:00:07.123>
> Temporal.PlainDateTime.from('2025-04-03T05:07.123')
Temporal.PlainDateTime <2025-04-03T05:00:07.123>

These strings are disallowed (technically they are valid ISO 8601 sec. 5.3.1.4.b, but only by agreement between the interchange parties, which we do not agree to in the case of fractional minutes or hours.)

There is most likely a testing gap here, we should cover that clock time parts such as 05:07.123 are rejected, as well as clock time parts such as 12.5 which the polyfill correctly rejects already.