feat(core): add ai playground feature flag and remove model switch feature flag (#12934)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added support for the "AI Playground" experimental feature, including
new settings and localization entries.

* **Refactor**
* Renamed configuration and service references from "Model Switch" to
"Playground" across the AI chat and playground interfaces.
* Updated feature flag from "enable_ai_model_switch" to
"enable_ai_playground" for consistency.

* **Bug Fixes**
* The "Model" submenu in AI chat preferences is now always visible,
simplifying menu options.

* **Chores**
  * Removed outdated Claude model options from the chat prompt.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/prompt/prompts.ts

@@ -1650,10 +1650,6 @@ const CHAT_PROMPT: Omit<Prompt, 'name'> = {
     'gpt-4.1',
     'o3',
     'o4-mini',
-    'claude-opus-4-20250514',
-    'claude-sonnet-4-20250514',
-    'claude-3-7-sonnet-20250219',
-    'claude-3-5-sonnet-20241022',
     'gemini-2.5-flash',
     'gemini-2.5-pro',
     'claude-opus-4@20250514',