Clamp rounded fractional seconds to the field width

[vineethsaivs]

DateTimeFormat.format_frac_seconds can render a fractional-seconds field that is wider than its declared number of digits.

The method rounds the microseconds to the requested precision:

value = self.value.microsecond / 1000000
return self.format(round(value, num) * 10**num, num)

round(value, num) can carry up to a whole second (for example 0.999 rounds to 1.0). Then 1.0 * 10**num == 10**num, which has num + 1 digits, and self.format ('%0*d' % (num, value)) prints all of them. Babel intentionally does not carry into the seconds field, so the fractional field simply overflows its width.

from datetime import time
from babel.dates import DateTimeFormat

DateTimeFormat(time(1, 2, 3, 990000), locale='en_US')['S']    # '10'  (expected '9')
DateTimeFormat(time(1, 2, 3, 999500), locale='en_US')['SS']   # '100' (expected '99')

A single S digit must be 0 to 9, but it can render 10.

Fix: clamp the rounded value to the largest in-field value so it stays within the declared width:

frac = min(int(round(value, num) * 10**num), 10**num - 1)
return self.format(frac, num)

This only changes the overflow case (the value is clamped to all nines, the closest representation without carrying into seconds). Every existing test_fractional_seconds expectation, including the deliberately rounded ['SSSS'] == '0346', is unchanged. Added a regression test covering the S, SS, and SSSS overflow inputs.