Summary

The rerank() function now accepts plain model strings in addition to RerankingModel objects, matching the behavior of generateText, embed, and other core functions.

// Before: required provider instance
const result = await rerank({
  model: gateway.rerankingModel('cohere/rerank-v3.5'),
  ...
});

// After: plain strings also work
const result = await rerank({
  model: 'cohere/rerank-v3.5',
  ...
});

• Add string to RerankingModel type union
• Add resolveRerankingModel() to resolve-model.ts (follows video model pattern — uses raw global provider since rerankingModel is not yet on ProviderV4 interface)
• Wire resolution into rerank() function

Test plan

• TypeScript compilation passes
• resolveRerankingModel tests: v4 passthrough, v3→v4 conversion, string via gateway, string via global provider, unsupported provider error, unsupported version error
• Existing rerank tests pass