Validate OpenAPI 3.0 Specification

Problem

While starting development on #590894 (closed), it was identified that our OpenAPI 3.0 specification is not valid.

Steps to Reproduce

1. Install any OpenAPI 3.0 validator (e.g. Scalar CLI, Swagger Editor)
2. Validate the YAML specification
3. Validate the converted JSON specification
4. Confirm both specifications are invalid

Solution

Implement OpenAPI 3.0 specification validation in CI/CD.

Acceptance Criteria

• Choose tooling for OpenAPI specification validation
• Resolve all validation errors
• Add validation in gitlab-org/gitlab for openapi_v3.yaml (Won't be done: !225252 (comment 3125408195))
• Add validation in gl-infra/api-docs for public/openapi_v3.json