fix: streaming for `Agent.createTask` (#788)

.changeset/four-ears-nail.md

apps/docs/blog/2024-04-26-v0.3.0.md

examples/agent/openai-task.ts

examples/agent/step_wise_openai.ts

packages/core/src/agent/anthropic.ts

packages/core/src/agent/base.ts

packages/core/src/agent/openai.ts

packages/core/src/agent/react.ts

packages/core/src/agent/types.ts