chore: lazy-load server plugin modules (kibana-security) (elastic#266614)

## Summary

Lazy-load server plugin implementations via `await import('./plugin')`
(and related entrypoint adjustments) from `server/index.ts`, consistent
with elastic#170856.

The migration in elastic#170856 reduced **startup time by ~4 seconds** in
measured scenarios. **Memory savings were not quantified but are
expected.**

## CODEOWNERS

- `examples/eso_model_version_example` → @elastic/kibana-security
- `packages/kbn-mock-idp-plugin` → @elastic/kibana-security

## Follow-up

- elastic#171080 — add an ESLint rule
so new plugins follow this pattern.

---

**Disclaimer:** This PR was prepared with AI assistance (Cursor /
Claude); please review thoroughly.

Author: afharo

examples/eso_model_version_example/server/index.ts

@@ -8,6 +8,8 @@
  */
 
 import type { PluginInitializer } from '@kbn/core/server';
-import { EsoModelVersionExample } from './plugin';
 
-export const plugin: PluginInitializer<void, void> = async () => new EsoModelVersionExample();
+export const plugin: PluginInitializer<void, void> = async () => {
+  const { EsoModelVersionExample } = await import('./plugin');
+  return new EsoModelVersionExample();
+};

packages/kbn-mock-idp-plugin/moon.yml

@@ -36,6 +36,7 @@ dependsOn:
   - '@kbn/core-http-server'
   - '@kbn/core-http-server-utils'
   - '@kbn/i18n'
+  - '@kbn/core'
 tags:
   - plugin
   - dev

packages/kbn-mock-idp-plugin/server/index.ts

@@ -7,10 +7,16 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
+import type { PluginInitializerContext } from '@kbn/core/server';
+
 import { ConfigSchema } from './config';
 
 export type { CreateSAMLResponseParams } from './plugin';
-export { plugin } from './plugin';
+
+export const plugin = async (initializerContext: PluginInitializerContext) => {
+  const { plugin: initPlugin } = await import('./plugin');
+  return initPlugin(initializerContext);
+};
 
 export const config = {
   schema: ConfigSchema,

packages/kbn-mock-idp-plugin/tsconfig.json

@@ -25,5 +25,6 @@
     "@kbn/core-http-server",
     "@kbn/core-http-server-utils",
     "@kbn/i18n",
+    "@kbn/core",
   ]
 }