Z.ai Coding Plan API endpoint support

[Raymond8196]

currently Z.ai provides two distinct API endpoints for different billing plans:

 From the official Z.ai documentation (https://docs.z.ai/):                                      
 > Note: When using the GLM Coding Plan, you need to configure the dedicated Coding endpoint     
 https://api.z.ai/api/coding/paas/v4 instead of the general endpoint https://api.z.ai/api/       
 paas/v4. The Coding API endpoint is only for Coding scenarios and is not applicable to          
 general API scenarios.

Current State
Cherry Studio currently has two Zhipu-related providers:
- 智谱 (zhipu): https://open.bigmodel.cn/api/paas/v4/ — domestic (China) general API ✅
- Z.ai: https://api.z.ai/api/paas/v4/ — international general API ✅
However, the Z.ai Coding Plan endpoint (https://api.z.ai/api/coding/paas/v4/) is not yet supported. Users with a GLM Coding Plan subscription cannot use it directly in Cherry Studio without manually changing the API Host.
Should we add a dedicated Z.ai Coding provider (similar to GitHub Copilot vs GitHub Models), or is there a better approach?