fix(workflow): forward stream response format

[iroiro147]

Summary

• forward the resolved WorkflowAgent.stream() response format into workflow model calls
• add a direct doStreamStep regression proving JSON response formats reach the mock model
• add a workflow package changeset

Tests

• pnpm --filter @ai-sdk/provider build
• pnpm --filter @ai-sdk/provider-utils build
• pnpm --filter @ai-sdk/gateway build
• pnpm --filter ai build
• pnpm --dir packages/workflow test:node -- do-stream-step.test.ts
• pnpm --dir packages/workflow type-check
• pnpm --dir packages/workflow build
• pnpm exec oxfmt --check packages/workflow/src/do-stream-step.ts packages/workflow/src/do-stream-step.test.ts .changeset/four-parrots-flow.md
• pnpm exec oxlint packages/workflow/src/do-stream-step.ts packages/workflow/src/do-stream-step.test.ts
• git diff --check

Refs #16565.