Power Query FillLastUpdated serialized with culture-specific time separator

[swmal]

Saving a workbook with Power Query connections on a thread with e.g. fi-FI culture produces invalid timestamps in the DataMashup metadata:

d2026-02-19T08.15.36.2283135Z (fi-FI, '.' as time separator) d2026-02-19T08:15:36.2283135Z (expected) Excel fails to parse this and shows: "String was not recognized as a valid DateTime".

Cause

ExcelPowerQueryMetaDataEntry.GetValueAsText() calls DateTime.ToString("yyyy-MM-ddTHH:mm:ss.fffffffZ") without CultureInfo.InvariantCulture. The : in the format string is a placeholder for the culture's time separator. The same issue affects int and double serialization in the same method.