feat: add JsonConverter for OpenApiSchema System.Text.Json serialization

[Mahdigln]

Description

Adds OpenApiSchemaJsonConverter, a System.Text.Json.JsonConverter<OpenApiSchema> that produces clean OpenAPI wire format output instead of the default verbose reflection-based serialization.

Type of Change

• New feature (non-breaking change which adds functionality)

Related Issue(s)

Closes #2299

Changes Made

• Added OpenApiSchemaJsonConverter in src/Microsoft.OpenApi/Converters/
• Supports all OpenAPI versions: 2.0, 3.0, 3.1 (default), 3.2
• Added Read (deserialization) support in addition to Write (serialization)
• Updated PublicAPI.Unshipped.txt with new public API surface

Testing

• Unit tests added/updated
• All existing tests pass

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Versions applicability

• My change applies to the version 2.X of the library, if so PR link:
• My change applies to the version 3.X of the library, if so PR link:
• I have evaluated the applicability of my change against the other versions above.

Additional Notes

Usage example:

var options = new JsonSerializerOptions();
options.Converters.Add(new OpenApiSchemaJsonConverter());

// Serialize
var json = JsonSerializer.Serialize(schema, options);

// Deserialize
var schema = JsonSerializer.Deserialize<OpenApiSchema>(json, options);

// Target a specific OpenAPI version
options.Converters.Add(new OpenApiSchemaJsonConverter(OpenApiSpecVersion.OpenApi3_0));

[Vincent Biret (baywet)]

Thanks for the contribution!

Let's a couple of comments to help get this merged

[Copilot]

Pull request overview

This PR introduces first-class System.Text.Json (de)serialization support for OpenApiSchema by adding a dedicated JsonConverter<OpenApiSchema> that emits/consumes OpenAPI wire-format JSON instead of default reflection-based output, addressing the need described in #2299.

Changes:

• Added OpenApiSchemaJsonConverter with version-targeted serialization (defaulting to OpenAPI 3.2) and deserialization support.
• Added unit tests validating basic serialize/deserialize and version-specific behavior.
• Updated public API baselines to include the new converter; also includes an encoding-only change to global.json.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/Microsoft.OpenApi/Converters/OpenApiSchemaJsonConverter.cs	New converter implementation for OpenApiSchema using OpenAPI wire-format I/O.
test/Microsoft.OpenApi.Tests/Converters/OpenApiSchemaJsonConverterTests.cs	New unit tests covering basic serialization/deserialization scenarios.
src/Microsoft.OpenApi/PublicAPI.Unshipped.txt	Declares the newly added public converter API surface.
global.json	Introduces a UTF-8 BOM at file start (encoding-only change).

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

[Vincent Biret (baywet)]

Thank you for making the changes!