Preserve OpenAPI 3.1 schema $ref siblings

[abramchikd]

Summary

OpenAPI 3.1 treats Schema Object keywords next to $ref as additional constraints. Previously, kin-openapi decoded these fields but dropped them during serialization and did not apply them consistently after reference resolution.

This change preserves schema $ref siblings across loading, validation, serialization, and reference internalization while keeping the existing SchemaRef.Value API and OpenAPI 3.0 resolution behavior.

Changes

• Decode sibling keywords through the ordinary Schema representation so they follow the same JSON/YAML round-trip rules as schemas without $ref.
• Resolve OpenAPI 3.1 siblings conjunctively by keeping the local schema as the effective value and adding the resolved target as an allOf conjunct.
• Serialize from the local sibling schema while retaining $ref as the reference boundary, avoiding target duplication and recursive marshaling.
• Handle self-references, longer cycles, boolean schemas, dynamic references, raw external schemas, and repeated resolution without losing constraints or mutating shared targets.
• Internalize references nested inside sibling schemas for both OpenAPI 3.0 and 3.1 documents without changing OpenAPI 3.0 validation semantics.
• Preserve use-site extensions and origin information.
• Centralize schema-child traversal so resolution and internalization cover every subschema field consistently.
• Make recursive instance tracking distinguish overlapping slices that share a backing address.
• Regenerate the reference types from the updated templates.

Compatibility

• SchemaRef.Value remains a non-nil *Schema after successful resolution.
• OpenAPI 3.0 continues to validate against the resolved target only.
• Serialization remains version-independent and now retains sibling fields supported by ordinary Schema serialization.

Testing

Added regression coverage for:

• internal and external references with sibling constraints;
• JSON and YAML serialization parity, including omitted values and pointer-backed zero values;
• conjunctive validation when sibling and target constraints overlap;
• self-references, multi-node cycles, boolean schemas, and $dynamicRef;
• idempotent resolution and isolation between references sharing one target;
• external-reference internalization, including nested references in sibling schemas and OpenAPI 3.0 compatibility;
• origin preservation and recursive validation of overlapping slices.

This pull request was created with the assistance of OpenAI Codex.