Fix node duration formatting when rounding crosses a unit boundary

[RonnyPfannschmidt]

Requested by Ronny · project thread

🤖 Written by Claude Opus 5.5 via Claude Code for the pytest maintainers; I prompted it, it did the work, I read it.

Merge strategy: squash.

Before: with console_output_style = times, a duration just under a unit boundary was printed with too many digits, e.g. 0.99996ms as 1000.0us and 59.9996s as 0m 60s. TestProgressOutputStyle::test_times failed on main because of this (windows-py311 job, output test_foo.py ..... 1000.0us).

After: the same durations print as 1.000ms and 1m 0s.

How: format_node_duration now formats the value first and only keeps a unit if the rounded number stays below that unit's limit, otherwise it moves to the next one. Minutes and hours are computed from the rounded whole seconds. New parametrized cases cover each boundary; all of them fail on main.

No library was used: humanize and humanfriendly produce long-form text ("999 milliseconds and 960 microseconds"), not the compact at-most-7-character column this output needs, and humanfriendly.format_timespan(59.9996) has the same rounding issue ("60 seconds").

• Tests added
• AI agent credited in Co-authored-by trailer
• Changelog entry

[RonnyPfannschmidt]

🤖 Written by Claude Opus 5.5 via Claude Code for the pytest maintainers; I prompted it, it did the work, I read it.

Why not use a library for this formatting? I checked the two common ones against the boundary values from the new tests:

seconds	humanize.precisedelta	humanize.naturaldelta	humanfriendly.format_timespan	this PR
0.00099996	1 millisecond	1 millisecond	0 seconds	1.000ms
0.0123	12 milliseconds and 300 microseconds	12 milliseconds	0.01 seconds	12.30ms
0.99996	999 milliseconds and 960 microseconds	999 milliseconds	1 second	1.000s
59.9996	59 seconds, 999 milliseconds and 600 microseconds	59 seconds	60 seconds	1m 0s
119.6	1 minute, 59 seconds and 600 milliseconds	2 minutes	1 minute and 59.6 seconds	2m 0s

• Both produce long-form text. The times progress column needs compact output of at most 7 characters.
• humanfriendly.format_timespan(59.9996) gives "60 seconds", which is the same rounding-across-a-boundary bug this PR fixes.
• Either one would add a runtime dependency to pytest to replace about 15 lines of code.

So the PR keeps the hand-written format_node_duration.

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