Rename MicroBenchmarks.Serializers.BinaryData to BinaryDataPayload

[DrewScoggins]

Description from copilot.

The serialization-test helper class MicroBenchmarks.Serializers.BinaryData shadowed the in-box System.BinaryData type (introduced in net6+ via System.Memory.Data, brought in transitively by Azure.Core).

Inside DataGenerator.cs (namespace MicroBenchmarks.Serializers) the unqualified typeof(BinaryData) always bound to the local helper class. Inside ReadJson.cs / WriteJson.cs (namespace System.Text.Json.Serialization.Tests) C# enclosing-namespace lookup found System.BinaryData first, so the [GenericTypeArguments(typeof(BinaryData))] attribute resolved to a DIFFERENT type than the Generate<T>() switch was checking. The mismatch was latent on main (where System.Memory.Data wasn't compile-visible to ReadJson.cs) but surfaced as System.NotImplementedException in DataGenerator.Generate<T>() whenever the reference closure changed (e.g. retargeting BenchmarkDotNet.Extensions from netstandard2.0 to net8.0).

Renaming the helper class to BinaryDataPayload removes the shadowing once and for all. No behavior change on main; future-proofs against reference-closure churn.

[Copilot]

Pull request overview

This pull request renames the microbenchmark serialization helper type MicroBenchmarks.Serializers.BinaryData to BinaryDataPayload to avoid shadowing System.BinaryData (net6+), which could cause GenericTypeArguments to bind to the wrong type and trigger runtime NotImplementedException in DataGenerator.Generate<T>().

Changes:

• Renamed the local helper model BinaryData to BinaryDataPayload in DataGenerator.cs.
• Updated DataGenerator.Generate<T>() and JSON source-generator registration to use BinaryDataPayload.
• Updated ReadJson<T> / WriteJson<T> benchmark generic type argument lists to reference BinaryDataPayload.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/benchmarks/micro/Serializers/DataGenerator.cs	Renames the helper model and updates generator/type registrations to eliminate System.BinaryData shadowing issues.
src/benchmarks/micro/libraries/System.Text.Json/Serializer/WriteJson.cs	Updates benchmark generic type argument from BinaryData to BinaryDataPayload.
src/benchmarks/micro/libraries/System.Text.Json/Serializer/ReadJson.cs	Updates benchmark generic type argument from BinaryData to BinaryDataPayload.

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