feat(ai-zai): add Z.AI adapter on the OpenAI chat completions base

.changeset/zai-chat-completions-adapter.md

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+---
+'@tanstack/ai-zai': minor
+---
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+Add the Z.AI adapter. It drives Z.AI's OpenAI-compatible Chat Completions API through `@tanstack/openai-base` and ships the GLM-5 and GLM-4 model catalog (`glm-5.3`, `glm-5.3-flash`, and `glm-5.2` first), `createZAIText`/`zaiText`, `createZAISummarize`/`zaiSummarize`, the branded `zaiWebSearchTool`, and a `zaiByok` descriptor at `@tanstack/ai-zai/byok` so a browser-supplied key wins over `ZAI_API_KEY`.
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+The adapter sets the coding-plan base URL when `coding: true` is passed. It sends `json_object` plus a schema system prompt because Z.AI rejects `json_schema`. Z.AI accepts a `tool_choice` of `auto` only, so `toolChoice: 'none'` sends no tools and `'required'` or a named tool warns once and falls back to `auto`. It streams `delta.reasoning_content` as AG-UI reasoning events.