feat(gateway-controller): scope immutable gateway write-blocking middleware to management routes only

Summary

When immutable gateway mode is enabled, the ImmutableGW.Middleware() in gateway/gateway-controller/pkg/immutable/loader.go blocks all POST, PUT, and DELETE requests globally. While all currently registered routes are management endpoints, this global placement creates a maintenance risk: future developers could inadvertently add non-management mutating endpoints (e.g., health checks, admin callbacks, webhook receivers) that would be unexpectedly blocked.

Proposed Fix

Either:

1. Create a dedicated route group for management endpoints in main.go and apply the middleware only to that group, or
2. Update the middleware body to check the request path (e.g., c.Request.URL.Path) and only enforce the write-block for the management route prefix(s).

Context

• Raised during review of PR #1678 (https://github.com/wso2/api-platform/pull/1678), comment: https://github.com/wso2/api-platform/pull/1678#discussion_r3055363018
• Deferred from PR #1678 by @renuka-fernando to be addressed in a follow-up.

Assignee

@renuka-fernando