[Python] Round float seconds to the nearest microsecond in Duration

[anishmehta24]

Duration(seconds) truncates instead of rounding when seconds is a float, so it can land one microsecond below the value passed in:

>>> Duration(2.000002).micros
2000001          # expected 2000002

seconds * 1000000 is a float multiplication that often lands just under the exact integer (2.000002 * 1e6 == 2000001.9999999998) and int() truncates it. #40257 fixed this for the Timestamp constructor; Duration.__init__ had the same line, so this uses round() there too, same as the Timestamp fix (micros keeps int(), matching it).

Test: DurationTest.test_constructor_float_rounds_to_nearest covers positive, negative, Duration.of, an exact value, and microsecond values round-tripped through float seconds. It fails on master. apache_beam/utils/timestamp_test.py 43 passed, yapf clean.

Fixes #40263

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[anishmehta24]

#40257 fixed the Timestamp half of this, so I've narrowed this PR to Duration, which still truncates the same way (Duration(2.000002).micros == 2000001 on master). Opened #40263 for it.