[Feature]: Treat built-in LLM provider templates as read-only in Gateway

Please select the area the issue is related to

Gateway

Please select the aspect the issue is related to

Aspect/API (API backends, definitions, contracts, interfaces, OpenAPI)

Suggested Feature

Built-in LLM provider templates should be treated as read-only. Users must not be able to update or delete them from any surface.

In the Platform API / control plane, PUT / PATCH / DELETE on a built-in template are blocked, as we treat them as read-only templates.But in the Gateway REST APIs, PUT / DELETE have no such guard, built-in templates can be updated or deleted. Also, when the gateway is restarted, those changes are lost during reseeding (deleted built-ins are re-created).
Since built-in templates are shipped and maintained by WSO2, modifying or deleting them should be restricted.

Proposed Solution

• Block update and delete operations on built-in templates. Requests must be rejected with a clear, actionable error message rather than failing silently or partially applying.

Related Issues

No response

Steps to Verify

• Design Document — A detailed design document has been created and reviewed, covering architecture, data flow, and edge cases.
• Design Mail — A design summary email has been sent to relevant stakeholders for awareness and feedback.
• Code Review — All code changes have been peer-reviewed and approved according to the project's review standards.
• Testing Complete — Adequate unit, integration, and/or end-to-end tests have been written and are passing.
• Documentation Review — User-facing and/or developer documentation has been updated to reflect the new feature and reviewed.
• Feature Complete — The feature is fully implemented, all checklist items above are done, and it is ready for release.

Start by locating the Gateway REST API handlers for PUT and DELETE operations on LLM provider templates, then compare their behavior with the existing Platform API/control-plane guards for built-in templates. Add coverage for rejecting updates and deletions with clear, actionable errors while allowing non-built-in templates to remain manageable. Done means the Gateway no longer changes built-in templates and tests cover both operations.