format-by-name-rules does not set the deserialization format string for datetime

[fengzhou-msft]

In OperationalInsights, using below config:

format-by-name-rules:
   'lastSkuUpdatedOn': 'datetime'

Actual behavior:
The generated deserialization code will use:
lastSkuUpdate = property.Value.GetDateTimeOffset();

This calls the JsonElement.GetDateTimeOffset() method that only supports ISO 8601 format.

This caused issue: Azure/azure-sdk-for-net#33536

Expected behavior:
The generated code should have a datetime format string:
lastSkuUpdate = property.Value.GetDateTimeOffset("O");

This calls an extension method:

autorest.csharp/src/assets/Generator.Shared/JsonElementExtensions.cs

Lines 66 to 71 in c8280c0

	public static DateTimeOffset GetDateTimeOffset(in this JsonElement element, string format) => format switch
	{
	"U" when element.ValueKind == JsonValueKind.Number => DateTimeOffset.FromUnixTimeSeconds(element.GetInt64()),
	// relying on the param check of the inner call to throw ArgumentNullException if GetString() returns null
	_ => TypeFormatters.ParseDateTimeOffset(element.GetString()!, format)
	};

It treats the JsonElement as Int64 (Unix Timestamp) or string and the string case can support multiple formats despite the "O" format (ISO8601) being passed in.

autorest.csharp/src/assets/Generator.Shared/TypeFormatters.cs

Lines 125 to 132 in c8280c0

	public static DateTimeOffset ParseDateTimeOffset(string value, string format)
	{
	return format switch
	{
	"U" => DateTimeOffset.FromUnixTimeSeconds(long.Parse(value, CultureInfo.InvariantCulture)),
	_ => DateTimeOffset.Parse(value, CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal)
	};
	}

[ArthurMa1978]

Have another issue #2524 opened to track all unsupported format.