fix(csharp): only generate XML serialization helpers when a model has XML usage

[Copilot]

The XML WriteObjectValue<T> extension method emits a case IPersistableModel<T> that calls ModelReaderWriter.Write(..., <Namespace>Context.Default). When the generated ModelReaderWriterContext registers no buildable types, the source generator fails to produce .Default, breaking compilation (observed in Azure.Storage.Blobs.Batch).

Root cause: InputLibrary.GetHasXmlModelSerialization() treated an enum with XML usage as XML serialization support. Enums do not implement IPersistableModel and are never registered with the context, so an XML-usage enum alone enabled the XML helpers while leaving the context empty.

Changes

• InputLibrary.GetHasXmlModelSerialization: only consider models (which implement IPersistableModel); removed the enum branch. This gates all XML serialization helpers — including WriteObjectValue and the BinaryContentHelper XML overload — on the presence of at least one XML model. Legitimate XML enums are already covered, since such enums only exist as properties of an XML model.
• Tests: added ValidateXmlMethodsAreNotGeneratedWhenOnlyEnumHasXmlUsage, asserting no XmlWriter/XElement helpers are emitted when only an enum carries XML usage.

// Before: enum-only XML usage returned true -> XML helpers emitted, empty context -> build break
// After: only models with XML usage count
foreach (var model in InputNamespace.Models)
{
    if (model.Usage.HasFlag(InputModelTypeUsage.Xml))
    {
        return true;
    }
}
return false;

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commit: bebeeef