Update to Microsoft.OpenApi 2.0.0-preview.18

[captainsafia]

This pull request updates the Microsoft.OpenApi and related libraries to version 2.0.0-preview.18 and replaces the usage of OpenApiAny with JsonNodeExtension across the codebase. It also removes unused using Microsoft.OpenApi.Any statements in several files. These changes improve compatibility with the updated library version and streamline the code.

Dependency Updates:

• Updated MicrosoftOpenApiVersion and MicrosoftOpenApiYamlReaderVersion to 2.0.0-preview.18 in eng/Versions.props.

Code Refactoring:

• Replaced OpenApiAny with JsonNodeExtension for schema extensions in multiple files, such as TransformersBenchmark.cs, CustomSchemaTransformerTests.cs, and SchemaTransformerTests.cs. This improves alignment with the updated library. [1] [2] [3]

Cleanup:

• Removed unused using Microsoft.OpenApi.Any statements and replaced them with using Microsoft.OpenApi.Extensions where necessary in both source and test files. [1] [2] [3]

Test Updates:

• Updated test assertions to use JsonNodeExtension instead of OpenApiAny to validate schema extensions in test files, such as SchemaTransformerTests.cs and OpenApiSchemaReferenceTransformerTests.cs. [1] [2]

These changes ensure compatibility with the latest library version and remove deprecated or unused code.

[mikekistler]

Looks good! 👍