refactor: improve agent type (#694)

examples/agent/multi_document_agent.ts

examples/agent/openai.ts

examples/agent/query_openai_agent.ts

examples/agent/react_agent.ts

examples/agent/step_wise_openai.ts

examples/agent/step_wise_query_tool.ts

examples/agent/step_wise_react.ts

examples/agent/stream_openai_agent.ts

examples/agent/wiki.ts

examples/toolsStream.ts

packages/core/src/Settings.ts

packages/core/src/agent/openai/base.ts

packages/core/src/agent/openai/utils.ts

packages/core/src/agent/openai/worker.ts

packages/core/src/agent/react/base.ts

packages/core/src/agent/react/worker.ts

packages/core/src/agent/types.ts

packages/core/src/agent/utils.ts

packages/core/src/llm/LLM.ts

packages/core/src/llm/base.ts