Fix format_duration() truncating each timestamp before subtracting - #3536

format_duration() truncated both operands before subtracting them, so the fractional part is lost twice: int(1700000002.1) - int(1700000000.9) is 2 although only 1.2 s elapsed.

get_html_report() feeds it stats.start_time and stats.last_request_timestamp (locust/html.py:99), so the duration in the HTML report is a whole second too long whenever start_time has the larger fractional part, and two runs of identical length can be displayed differently depending on where the whole-second boundary falls.

This commit truncates once, on the difference. Inputs that already are whole seconds format exactly as before, so the values in the existing dates_checks table are unchanged.

Fixes #3535

How was it checked

On master at c6c51480, with the new test parametrization added and the source line untouched (baseline):

locust/test/test_date.py  ->  3 failed, 2 passed, 30 skipped
FAILED ...[1700000000.9-1700000002.1-1 second]   '2 seconds' != '1 second'
FAILED ...[1700000000.8-1700000001.4-0 seconds]  '1 second' != '0 seconds'
FAILED ...[1700000000.7-1700086401.3-1 day]      '1 day and 1 second' != '1 day'

after the one-line change:

locust/test/test_date.py  ->  5 passed, 30 skipped
ruff check locust/util/date.py locust/test/test_date.py  ->  All checks passed
ruff format --check (same files)  ->  2 files already formatted

locust/test/test_web.py -k "report or duration or timestamp" gives 7 failed, 1 passed both before and after this change on this Windows machine (pre-existing GET /stats/report 500s because the built web UI assets are missing locally), so no difference is introduced there.

Drafted with AI assistance in this session; no human reviewed it before submission.