feat: add JsonConverter for OpenApiSchema System.Text.Json serialization - #2915

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));