feat(zai): add Z.AI provider (#19630)

## Summary

- add the first-party `@ai-sdk/zai` provider for Z.AI GLM chat
completions
- support streaming, reasoning, tool calls, multimodal URL inputs,
Z.AI-specific options, error handling, and workflow serialization
- add Node and Edge tests, examples, provider documentation, workspace
integration, and a major changeset

Closes #17254

## Test plan

- `pnpm --filter @ai-sdk/zai build`
- `pnpm --filter @ai-sdk/zai test`
- `pnpm --filter @ai-sdk/zai type-check`
- `pnpm type-check:full`
- `pnpm check`
- `npm pack --dry-run` with a temporary npm cache

Live API examples were not run because `ZAI_API_KEY` is not configured
in this environment.