Add support for OpenAPI discriminator in unions

[andrewzolotukhin]

Description

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactor / internal improvement

Checklist

• I've added tests for my changes
• I've run npm run lint and fixed any issues
• I've run npm run test and all tests pass
• I've added a changeset (npx changeset) if this changes package behavior

[Copilot]

Pull request overview

Adds end-to-end support for emitting the OpenAPI discriminator keyword for discriminated unions across @cleverbrush/schema introspection and @cleverbrush/schema-json / @cleverbrush/server-openapi spec generation, with accompanying tests and documentation updates.

Changes:

• Expose discriminatorPropertyName via UnionSchemaBuilder.introspect() when a union is detected as discriminated.
• Emit discriminator: { propertyName, mapping? } alongside anyOf in toJsonSchema() for discriminated unions (mapping when branches are $refs).
• Add tests and update docs/site content to document and validate discriminator behavior in generated OpenAPI/JSON Schema.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
website/app/server-openapi/page.tsx	Adds docs note that discriminated unions emit OpenAPI discriminator.
website/app/schema/sections/discriminated-unions.tsx	Documents JSON Schema/OpenAPI output for discriminated unions, including discriminator.
website/app/schema-json/page.tsx	Updates feature table to mention discriminator emission for unions.
libs/server-openapi/src/generateOpenApiSpec.test.ts	Adds OpenAPI-spec tests asserting discriminator emission and $ref mapping behavior.
libs/server-openapi/README.md	Documents discriminated unions and discriminator output in generated OpenAPI specs.
libs/schema/src/builders/UnionSchemaBuilder.ts	Adds discriminatorPropertyName to union introspection.
libs/schema/src/builders/UnionSchemaBuilder.test.ts	Adds tests for discriminatorPropertyName introspection behavior.
libs/schema/src/builders/SchemaBuilder.ts	Rewords .schemaName() docs to emphasize metadata role and broader tooling use.
libs/schema-json/src/types.ts	Extends JSON schema node typing to allow optional discriminator alongside anyOf.
libs/schema-json/src/toJsonSchema.ts	Emits discriminator (and optional mapping) for discriminated unions.
libs/schema-json/src/toJsonSchema.test.ts	Adds tests for discriminator emission, nullable unions, and $ref mapping.
libs/schema-json/README.md	Documents discriminator emission behavior for discriminated unions.
.changeset/discriminated-union-discriminator.md	Changeset describing the new feature and affected packages.

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