Adds first-class round-trip assertions for System.Text.Json, which is the serializer many users rely on today and which was missing alongside the existing XML and data contract helpers.

The implementation adds BeJsonSerializable() / BeJsonSerializable<T>() overloads, supports custom equivalency configuration, and accepts JsonSerializerOptions so callers can use custom converters and other serializer settings. The comparison follows System.Text.Json semantics by checking properties by default and excluding [JsonIgnore] members from the round-trip equivalency check.

The JSON assertion is only compiled on NET6_0_OR_GREATER, matching the existing JSON equivalency API surface.

The specs cover successful round-tripping, serializer failures, ignored members, custom converter scenarios, and null option guards. The unreleased notes and serialization docs were updated to document the new assertion and ignored-member behavior.

Closes #3264

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
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• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
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  • Please also run ./build.sh --target spellcheck or .\build.ps1 --target spellcheck before pushing and check the good outcome

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