[Feat] Add Z.AI and Z.AI Coding Plan inference providers

[pridemusvaire]

Summary

Adds Z.AI and Z.AI Coding Plan as first-class BYOK inference providers, with an International / China region select on connect.

• Catalog ids: zai, zai-coding-plan
• Model ids: zai/glm-5.2, zai-coding-plan/glm-5.2 (region-independent)
• Env: ZAI_API_KEY + ZAI_REGION, ZAI_CODING_PLAN_API_KEY + ZAI_CODING_PLAN_REGION (global | china, default global)
• Inference gateway uses models.dev v4 bases and /chat/completions paths (not OpenAI /v1/...)
• Connect UI supports selectable additionalEnvFields.options (region select)

Test plan

• Types + API gateway unit tests
• Pre-push: check-types, lint, knip
• CI green on this PR

[roomote-roomote]

No code issues found. See task

_{Reviewed c55f7bc}