test(zai): derive user-agent version (#19783)

## Summary

- derive the expected ZAI user-agent suffix from the exported VERSION
value
- remove the hard-coded test version so package releases cannot stale
the assertion
- keep coverage that requests identify the ZAI provider and its current
package version

## Testing

- pnpm --filter @ai-sdk/zai... build
- pnpm --filter @ai-sdk/zai test:node (17/17)
- pnpm --filter @ai-sdk/zai test:edge (17/17)
- pnpm --filter @ai-sdk/zai type-check
- pnpm check
- pnpm type-check:full

No changeset is needed because this is a test-only change.