Generate OpenAPI specification and client libraries for Identifiers API

As an integration developer, 
I want an OpenAPI specification and generated client libraries 
for the Identifiers API, 
so that I can reliably integrate with 
the CHRIS/CAMSIS → CRSid mapping endpoints using stable,
versioned artifacts.

Background

• The Identifiers API exposes CHRIS/CAMSIS → CRSid mapping endpoints (POST/GET).
• We need a machine-readable API contract and generated client libraries published as build artifacts for reuse by downstream services.

Acceptance Criteria

• An OpenAPI spec for the Identifiers API is defined in-repo and kept under version control.
• All identifier mapping endpoints (current and future) are described in the OpenAPI spec, including request/response schemas and error formats.
• CI validates the OpenAPI spec on every pipeline run and fails the pipeline on invalid changes.
• CI generates client libraries from the OpenAPI spec.
• Generated client libraries are published as versioned build artifacts and are reproducible from CI.
• Basic usage of the generated client(s) is documented (link + minimal code example) for integration developers.