refactor(powershell): drop redundant millisecond subtraction in backfill end date

The -Backfill default end date subtracted a millisecond to land on the last
day of the window, then had the time truncated away by .Date a few lines
later. For a midnight start date -- always the case here, since both
-StartDate sources are midnight -- AddMonths(1).AddMilliseconds(-1).Date and
AddMonths(1).AddDays(-1) are identical, so this is a no-op.

Using AddDays(-1) also makes the two default-end-date paths read the same;
the non-backfill branch below already computes it that way.

Note this is not the same as 'the last day of the month': with an explicit
-StartDate that is not the first (for example -StartDate '2026-03-15'
-Backfill 2), the window stays one month from the start date and ends
2026-04-14, not 2026-03-31.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Roland Krummenacher

src/powershell/Public/Start-FinOpsCostExport.ps1

@@ -119,7 +119,7 @@ function Start-FinOpsCostExport
         # If -EndDate is not set, assume 1 month
         if (-not $EndDate)
         {
-            $EndDate = $StartDate.AddMonths(1).AddMilliseconds(-1)
+            $EndDate = $StartDate.AddMonths(1).AddDays(-1)
         }
 
         # Move start date to account for the backfill period