Fix two windows-latest test flakes: TestNPOI1469 rounding and POIDataSamples.GetFile sharing mode

[ken-swyfft]

windows-latest on master has been red on its last two pushes (runs 33473168767 on 2026-09-01 and 34364474895 on 2026-09-09) for a single test each time, TestNPOI1469, and a fork of this repo hit a second single-test failure on the same job. Both are test-infrastructure races, not library bugs. This PR fixes both. Every other test in those runs passed on both TFMs, so with these two changes the Windows job should go green.

1. TestNPOI1469 ("Row mismatch (0)", testcases/main/HSSF/UserModel/TestCellStyle.cs)

The test formats its start time with ToString("HH:mm:ss"), which truncates, and compares against DataFormatter, which rounds the Excel OLE-date double. Whenever DateTime.UtcNow lands in the upper half of a second the two disagree by one second. Truncating the start time to whole seconds makes both paths agree. This fix has been running in the swyfft-insurance fork since July with no recurrence.

2. TestDetectAsPOIFS (IOException: SampleSS.xls ... being used by another process)

POIDataSamples.GetFile() opens sample files with FileMode.OpenOrCreate, which means FileAccess.ReadWrite plus FileShare.Read. Nuke runs the net472 and net10.0 test hosts in parallel, so while TestHSSFWorkbook.CloseDoesNotModifyWorkbook holds SampleSS.xls through that handle, the other host's ConfirmIsPOIFS reader is refused. Linux has no such sharing rule, which is why ubuntu-latest never sees it. No caller writes through this handle: the one Write() that follows it is asserted to throw on a read-only filesystem, and the WriteFileSystem() calls in TestNPOIFSFileSystem target temp files. Opening read-only with FileShare.ReadWrite removes the race without changing any test's behaviour. Seen on swyfft-insurance/npoi run 34372163455 (attempt 1); the re-run of the same commit passed.

Verified locally on this branch: NPOI.TestCases builds on net10.0, and TestNPOI1469, TestOfficeXMLException, and CloseDoesNotModifyWorkbook pass. Both changes are confined to testcases/main.

(PR opened by Claude, an AI assistant, on Ken's behalf.)

[ken-swyfft]

One more data point for the GetFile() half of this PR: #1868 (a csproj-metadata-only change on top of current master) just failed windows-latest on a third test with the same signature, Bug59058 in NPOI.OOXML.TestCases (net10.0):

System.IO.IOException : The process cannot access the file '...\testcases\test-data\document\bug59058.docx' because it is being used by another process

That test also goes through POIDataSamples.GetDocumentInstance().GetFile(), so the fix here covers it as well. That makes three distinct tests (TestDetectAsPOIFS, Bug59058, plus whatever GetFile caller loses the race next time) hitting the same OpenOrCreate handle.

(Comment by Claude, an AI assistant, on Ken's behalf.)

[tonyqus]

LGTM