Fix missing Edm.Time (TimeSpan) support in OData v2 filter expressions

[Copilot]

TimeSpan properties (mapped from Edm.Time in OData v2) could not be used in LINQ filter expressions — AppendByValueType had no handler for TimeSpan, causing NotSupportedException. No ToODataFilter overload existed for TimeSpan in FilterHelper.

Changes

• FilterHelper.cs — Added ToODataFilter(TimeSpan, ODataVersion) and nullable overload. Produces time'PT9H18M39S' for v2/v3 and duration'PT9H18M39S' for v4, using XmlConvert.ToString for ISO 8601 duration serialization.
• FilterExpressionVisitor.cs — Added TimeSpan branch in AppendByValueType (TypeCode.Object), delegating to the new FilterHelper overload.
• FilterExpressionTests.cs — Added Duration/OptionalDuration (TimeSpan/TimeSpan?) to TestProduct; added #region TimeSpan Tests covering v2/v4 literals, comparison operators, nullable, and inline constants.
• ODataTimeSpanConverterTests.cs — Added "PT09H18M39S" (the exact server-reported value) as an inline theory case; added OData v2 response round-trip tests for both non-nullable and nullable Edm.Time properties.

Example

// Previously threw NotSupportedException
Expression<Func<MyEntity, bool>> expr = e => e.Duration == new TimeSpan(9, 18, 39);
var filter = visitor.ToFilter(expr, ODataVersion.V2);
// Now produces: (Duration eq time'PT9H18M39S')

var filter4 = visitor.ToFilter(expr, ODataVersion.V4);
// Now produces: (Duration eq duration'PT9H18M39S')