Refactors Apache.Arrow.Variant into two assemblies

[CurtHagenlocher]

What's Changed

Refactors Apache.Arrow.Variant into two assemblies -- one for the base type and one for JSON conversion. The dependency on System.Text.Json moves to the second assembly.

[Copilot]

Pull request overview

Refactors the Variant implementation by separating JSON-related functionality into a new Apache.Arrow.Operations assembly, allowing Apache.Arrow.Variant to drop its direct dependency on System.Text.Json while preserving JSON parsing/writing capability via the new assembly.

Changes:

• Introduces Apache.Arrow.Operations (with Apache.Arrow.Operations.Json) to host JSON reader/writer/converter functionality.
• Moves streaming value emission logic into a new VariantValueWriter, and updates VariantBuilder.Encode to use it.
• Updates tests/benchmarks/solutions to reference the new assembly and namespaces.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/Apache.Arrow.Variant.Tests/VariantSqlDecimalTests.cs	Updates JSON namespace usage to Apache.Arrow.Operations.Json.
test/Apache.Arrow.Variant.Tests/Apache.Arrow.Variant.Tests.csproj	Adds project reference to Apache.Arrow.Operations.
test/Apache.Arrow.Variant.Benchmarks/EncodingBenchmarks.cs	Switches streaming JSON encode benchmarks to VariantJsonReader.Parse.
test/Apache.Arrow.Variant.Benchmarks/Apache.Arrow.Variant.Benchmarks.csproj	Adds project reference to Apache.Arrow.Operations.
test/Apache.Arrow.Operations.Tests/VariantJsonTests.cs	Moves/renames JSON tests into the new operations test project + namespace.
test/Apache.Arrow.Operations.Tests/Apache.Arrow.Operations.Tests.csproj	Adds new test project for Apache.Arrow.Operations.
src/Apache.Arrow.Variant/VariantValueWriter.cs	Adds streaming writer for variant value bytes (used by builder and JSON reader).
src/Apache.Arrow.Variant/VariantBuilder.cs	Refactors encoding to use VariantValueWriter; removes JSON encoding from this assembly.
src/Apache.Arrow.Variant/Properties/AssemblyInfo.cs	Adds InternalsVisibleTo for Apache.Arrow.Operations.
src/Apache.Arrow.Variant/Json/VariantJsonReader.cs	Removes old JSON reader from the Variant assembly.
src/Apache.Arrow.Variant/Apache.Arrow.Variant.csproj	Removes System.Text.Json dependency.
src/Apache.Arrow.Operations/Properties/AssemblyInfo.cs	Adds InternalsVisibleTo for operations tests.
src/Apache.Arrow.Operations/Json/VariantJsonWriter.cs	Moves JSON writer into Apache.Arrow.Operations.Json and references Variant core types.
src/Apache.Arrow.Operations/Json/VariantJsonReader.cs	Adds JSON-to-variant binary parser implemented with VariantValueWriter.
src/Apache.Arrow.Operations/Json/VariantJsonConverter.cs	Moves JsonConverter<VariantValue> into operations assembly/namespace.
src/Apache.Arrow.Operations/Apache.Arrow.Operations.csproj	Adds new assembly with System.Text.Json dependency and reference to Variant core.
Apache.Arrow.sln	Adds new projects and solution folder structure (src/test), plus additional build configs.
Apache.Arrow.Tests.slnf	Includes the new Apache.Arrow.Operations.Tests project in the test solution filter.

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[adamreeve]

Looks good to me, just some minor suggestions that you could choose to ignore