ZAI Coding Plan China provider integration

What Changed

Added 'zai-coding-cn' provider variant for China region pointing to https://open.bigmodel.cn/api/coding/paas/v4 with ZAI_CODING_CN_API_KEY environment variable. Includes five GLM models (GLM-4.5-Air, GLM-4.7, GLM-5-Turbo, GLM-5.1, GLM-5V-Turbo) with reasoning support and context windows.

Operator Implication

Expands provider support to China region users via BigModel API endpoint. Operators can now select ZAI Coding Plan China variant for region-specific deployments.

Receipt

• Commit 51df39b 'feat(ai): add ZAI Coding Plan China provider' adds 'zai-coding-cn' endpoint with 5 GLM models and corresponding environment variable