Assert dates in content editing integration tests to millisecond only

[AndyButland]

Have seen some tests failing on main due to date comparisons going beyond the millisecond and see dates as different when we should consider them the same.

E.g.:

  Failed Updating_Single_Variant_Property_Does_Not_Change_Update_Dates_Of_Other_Variants
  Error Message:
     Expected: 2025-06-07 00:32:57.9074126
  But was:  2025-06-07 00:32:57.907

This fixes that for the two cases seen on a recent pipeline run. Maybe we'll need to cherry-pick this into release/16.0 and perhaps branches for older versions.

[Copilot]

Pull Request Overview

This PR ensures date comparisons in content editing integration tests are truncated to millisecond precision and adds corresponding support in the DateTimeExtensions helper.

• Added a Millisecond enum member and updated TruncateTo to handle millisecond truncation.
• Updated integration tests to call .TruncateTo(DateTruncate.Millisecond) in date assertions.
• Fixed a typo in a test method name (Vaiants → Variants).

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
tests/Umbraco.Tests.Integration/Umbraco.Infrastructure/Services/ContentEditingServiceTests.Update.cs	Renamed test method, updated Assert calls to truncate dates to milliseconds
src/Umbraco.Core/Extensions/DateTimeExtensions.cs	Added DateTruncate.Millisecond and reworked TruncateTo logic to include milliseconds

Comments suppressed due to low confidence (1)

src/Umbraco.Core/Extensions/DateTimeExtensions.cs:70

• Consider adding unit tests specifically for TruncateTo(DateTruncate.Millisecond) to verify that millisecond truncation behaves correctly.

return new DateTime(dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, dt.Second, dt.Millisecond, dt.Kind);