Create Activout.RestClient.Test.Json project to test both JSON implementations

[Copilot]

This PR creates a new Activout.RestClient.Test.Json project that runs the same JSON-dependent test cases against both Activout.RestClient.Json (System.Text.Json) and Activout.RestClient.Newtonsoft.Json implementations.

Changes Made

• New test project: Created Activout.RestClient.Test.Json with xUnit framework
• Dual JSON support: Added project references to both JSON implementation libraries
• Parameterized tests: Used [Theory] and [InlineData] attributes to run tests with both implementations
• Shared test models: Created implementation-agnostic models and interfaces in the MovieReviews namespace
• Test coverage: Ported key tests from existing projects:
  • RestClientTests - Core REST client functionality with JSON serialization/deserialization
  • SerializationOrderTest - Tests serialization ordering behavior
  • SimpleValueObjectTest - Tests simple value object handling

Implementation Details

The solution uses a JsonImplementation enum to parameterize tests across both implementations:

public enum JsonImplementation
{
    SystemTextJson,
    NewtonsoftJson
}

[Theory]
[InlineData(JsonImplementation.SystemTextJson)]
[InlineData(JsonImplementation.NewtonsoftJson)]
public async Task TestPostJsonAsync(JsonImplementation jsonImplementation)
{
    // Test runs with both implementations
}

Each test creates the appropriate REST client builder based on the implementation:

return jsonImplementation switch
{
    JsonImplementation.SystemTextJson => builder.WithSystemTextJson(),
    JsonImplementation.NewtonsoftJson => builder.WithNewtonsoftJson(),
    _ => throw new ArgumentOutOfRangeException(nameof(jsonImplementation))
};

Test Results

All 25 tests pass successfully with both JSON implementations, ensuring compatibility and consistent behavior across both libraries.

Fixes #81.

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