Fix XML serialization for nullable value types

[Copilot]

Generator was passing nullable value types directly to XML serialization methods instead of accessing their underlying .Value property, causing compilation errors.

Changes

• Modified CreateXmlWriteValueStatement to call NullableStructValue() on values before passing to WriteStringValue and WriteBase64StringValue
• Modified CreateXmlSerializeValueExpression to call NullableStructValue() before passing to CreateXmlSerializePrimitiveExpression, ensuring comprehensive coverage for all nullable value types
• Applies to all nullable value types including DateTimeOffset?, TimeSpan?, int?, bool?, byte[]?, etc.
• Added test CanSerializeCustomizedNullableDateTimeOffset to verify nullable properties customized via CodeGenMember attribute are correctly serialized

Example

Before:

if (Optional.IsDefined(PolicyStartsOn))
{
    writer.WriteStartElement("Start");
    writer.WriteStringValue(PolicyStartsOn, "O");  // Compile error: DateTimeOffset? != DateTimeOffset
    writer.WriteEndElement();
}

After:

if (Optional.IsDefined(PolicyStartsOn))
{
    writer.WriteStartElement("Start");
    writer.WriteStringValue(PolicyStartsOn.Value, "O");
    writer.WriteEndElement();
}

Non-nullable types unaffected - NullableStructValue() returns the value unchanged when not a nullable value type.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix Incorrect XML Serialization Code for Nullable DateTimeOffset Properties</issue_title>
<issue_description>See the storage library for an example. The Start and Expiry spec properties were customized to be DateTimeOffset. Our xml serialization code for this model is not correct, since we are not accessing the property's underlining value:

 if (Optional.IsDefined(PolicyStartsOn))
 {
     writer.WriteStartElement("Start");
     writer.WriteStringValue(PolicyStartsOn, "O");
     writer.WriteEndElement();
 }
 if (Optional.IsDefined(PolicyExpiresOn))
 {
     writer.WriteStartElement("Expiry");
     writer.WriteStringValue(PolicyExpiresOn, "O");
     writer.WriteEndElement();
 }

We should be calling writer.WriteStringValue(PolicyStartsOn.Value, "O");.

In general, this applies to all nullable types.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix Incorrect XML Serialization Code for Nullable DateTimeOffset Properties #9735

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[jorgerangel-msft]

@copilot since this can apply to other types aside from DateTimeOffset, and this method returns the ValueExpression if the type is not nullable, lets apply this across all types when we write the value.

[Copilot]

Done in ed670a4. Applied NullableStructValue() to all value types by modifying CreateXmlSerializeValueExpression to call it before passing to CreateXmlSerializePrimitiveExpression. This ensures all nullable value types (int?, bool?, etc.) correctly access .Value.

[jorgerangel-msft]

@copilot lets also add an additional unit test to exercise the case where a datetime property is customized to be nullable via custom code.

[Copilot]

Added in ed670a4. Created test CanSerializeCustomizedNullableDateTimeOffset that verifies a DateTimeOffset property customized to be nullable via CodeGenMember attribute correctly uses .Value accessor in the generated XML serialization code.