Add SpectorTests for SpreadRecordNonDiscriminatedUnion3 scenario

[Copilot]

Adds missing test coverage for the SpreadRecordNonDiscriminatedUnion3 scenario from http-specs (Record<WidgetData2[] | WidgetData1>).

Changes

• Added two SpectorTest methods in AdditionalPropertiesTests.cs:
  • SpreadRecordNonDiscriminatedUnion3Get: Tests GET with heterogeneous union type additional properties
  • SpreadRecordNonDiscriminatedUnion3Put: Tests PUT with the same model

Implementation Details

The union type Record<WidgetData2[] | WidgetData1> generates IDictionary<string, BinaryData> where values can be arrays or single objects:

// Array serialization - arrays don't implement IJsonModel<T>
["prop1"] = BinaryData.FromObjectAsJson(new[] { 
    new WidgetData2("2021-01-01T00:00:00Z"),
    new WidgetData2("2021-01-01T00:00:00Z")
})

// Single object serialization
["prop2"] = ModelReaderWriter.Write(new WidgetData1(...))

Deserialization uses ToObjectFromJson<WidgetData2[]>() for arrays and ModelReaderWriter.Read<WidgetData1>() for single objects.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Adopt new Type/Property/additional-property scenarios</issue_title>
<issue_description>There are several new scenarios here that we would need to adopt by adding the corresponding SpectorTests.</issue_description>

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

• Fixes Adopt new Type/Property/additional-property scenarios #8946

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[JoshLove-msft]

@copilot the spector tests are failing. can you please investigate the issue?

[Copilot]

@copilot the spector tests are failing. can you please investigate the issue?

I've investigated the tests and found that:

1. The http-specs e2e tests for SpreadRecordNonDiscriminatedUnion3 are passing ✓
2. The C# SpectorTests I added compile successfully
3. The SpectorTests are being skipped locally because the generated client code is stubbed (which is expected for checked-in code)

Could you provide more details about which specific tests are failing in CI? Are you seeing:

• Build failures?
• Test execution failures?
• Coverage check failures?

This will help me identify and fix the exact issue.

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