Implement IJsonModel of DataFactoryElement<> for AOT compat

[live1206]

Resolves #50403

[Copilot]

Pull Request Overview

This PR implements the IJsonModel interface for DataFactoryElement to improve AOT (ahead-of-time) compatibility and adjust serialization behavior accordingly. Key changes include:

• Enhancing Utf8JsonWriterExtensions with a generic WriteObjectValue overload and modifying its switch cases to handle IJsonModel.
• Updating test logic in DataFactoryElementTests to use the new IPersistableModel-based serialization approach and modifying TestModel to implement IJsonModel.
• Refactoring serialization logic in DataFactoryKeyVaultSecret, DataFactoryElementJsonConverter, and DataFactoryElement.Serialization while introducing a new DataFactoryContext and adding a PackageReference for System.ClientModel.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sdk/core/Azure.Core/src/Shared/Utf8JsonWriterExtensions.cs	Updated method signature to include generic overload and added handling for IJsonModel.
sdk/core/Azure.Core.Expressions.DataFactory/tests/DataFactoryElementTests.cs	Adjusted serialization calls in tests and updated TestModel to implement IJsonModel.
sdk/core/Azure.Core.Expressions.DataFactory/src/DataFactoryKeyVaultSecret.Serialization.cs	Modified serialization logic to use IJsonModel for secret properties.
sdk/core/Azure.Core.Expressions.DataFactory/src/DataFactoryElementJsonConverter.cs	Removed original Serialize methods and replaced Write override with a NotImplementedException.
sdk/core/Azure.Core.Expressions.DataFactory/src/DataFactoryElement.cs	Converted to a partial class with additional warning suppressions.
sdk/core/Azure.Core.Expressions.DataFactory/src/DataFactoryElement.Serialization.cs	Implemented IJsonModel and IPersistableModel for DataFactoryElement.
sdk/core/Azure.Core.Expressions.DataFactory/src/DataFactoryContext.cs	Added a new context type for AOT-compatible model reading/writing.
sdk/core/Azure.Core.Expressions.DataFactory/src/Azure.Core.Expressions.DataFactory.csproj	Added PackageReference to System.ClientModel.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

Azure.Core.Expressions.DataFactory

Core shared class has been reverted.