Narrow APIKeyUpdateRequest schema to remove immutable fields from update endpoints

Summary

The APIKeyUpdateRequest schema currently inherits from APIKeyCreationRequest (via allOf), which exposes create-only/immutable fields (name, issuer, externalRefId) on update operations. The update flow in the gateway ignores these fields and copies them from the existing key, but the OpenAPI contract incorrectly documents them as writable.

Affected Endpoints

• PUT /rest-apis/{id}/api-keys/{apiKeyName}
• PUT /llm-providers/{id}/api-keys/{apiKeyName}
• PUT /llm-proxies/{id}/api-keys/{apiKeyName}

Required Changes

1. Introduce a dedicated APIKeyPatchRequest schema (or similar) that omits name, issuer, and externalRefId.
2. Update the requestBody schema references for all three update endpoints in gateway/gateway-controller/api/management-openapi.yaml to use the new narrowed schema.
3. Update the corresponding Go type aliases in gateway/gateway-controller/pkg/api/management/generated.go (e.g., UpdateAPIKeyJSONRequestBody, UpdateLLMProviderAPIKeyJSONRequestBody, UpdateLLMProxyAPIKeyJSONRequestBody) to reflect the new schema after regeneration.

Background

This was identified during code review in PR #1567 (adding API key management for LLM entities). Agreed to address in a follow-up PR for consistency across all API key update endpoints.

References:

• PR: https://github.com/wso2/api-platform/pull/1567
• Comment: https://github.com/wso2/api-platform/pull/1567#discussion_r3008377540
• Requested by: @Arshardh