Fix datetime last_modified rounding

[nightcityblade]

What do these changes do?

Makes StreamResponse.last_modified round datetime inputs the same way the numeric timestamp path already does, and adds a regression test for the sub-second case.

Are there changes in behavior for the user?

Yes. Datetime values with fractional seconds now round up consistently instead of truncating the fractional part.

Is it a substantial burden for the maintainers to support this?

No. This aligns two existing code paths that set the same header.

Related issue number

Fixes #5303

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES/ folder

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.24%. Comparing base (6ae358f) to head (2bfc91e).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.14   #13041   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files         135      135           
  Lines       48774    48779    +5     
  Branches     2613     2613           
=======================================
+ Hits        47916    47921    +5     
  Misses        678      678           
  Partials      180      180           

Flag	Coverage Δ
CI-GHA	98.29% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.06% <100.00%> (+<0.01%)	⬆️
OS-Windows	95.73% <100.00%> (-0.01%)	⬇️
OS-macOS	97.26% <100.00%> (+<0.01%)	⬆️
Py-3.10	97.45% <100.00%> (+<0.01%)	⬆️
Py-3.11	97.73% <100.00%> (+<0.01%)	⬆️
Py-3.12	97.81% <100.00%> (+<0.01%)	⬆️
Py-3.13	97.79% <100.00%> (+0.01%)	⬆️
Py-3.14	97.89% <100.00%> (-0.01%)	⬇️
Py-3.14t	96.89% <100.00%> (-0.01%)	⬇️
Py-pypy-3.11	96.74% <100.00%> (+<0.01%)	⬆️
VM-macos	97.26% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.06% <100.00%> (+<0.01%)	⬆️
VM-windows	95.73% <100.00%> (-0.01%)	⬇️
cython-coverage	38.47% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 72 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing nightcityblade:fix/issue-5303 (2bfc91e) with 3.14 (6ae358f)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩