Enable reflection-based serialization in System.Net.ServerSentEvents.Tests on Apple mobile

[kotlarmilos]

Description

This PR enables reflection-based serialization in System.Net.ServerSentEvents.Tests which should resolve trimming-related errors on Apple mobile platforms. The tests report the following exception:

Exception messages: System.InvalidOperationException : Reflection-based serialization has been disabled for this application. Either use the source generator APIs or explicitly configure the 'JsonSerializerOptions.TypeInfoResolver' property.   Exception stack traces:    at System.Text.Json.JsonSerializerOptions.ConfigureForJsonSerializer()
[06:00:34.9598940]    at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions , Type )
[06:00:34.9599000]    at System.Text.Json.JsonSerializer.GetTypeInfo[JsonElement](JsonSerializerOptions )

Changes

The JsonSerializerIsReflectionEnabledByDefault feature switch is turned off automatically when PublishTrimmed property is set.

Validation

The tests should pass in the ioslike pipeline https://dev.azure.com/dnceng-public/public/_build?definitionId=225&_a=summary.

[kotlarmilos]

/azp run runtime-extra-platforms

[dotnet-policy-service]

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[kotlarmilos]

Resolved in #103126.