refactor: unify response and agent response (#930)

.changeset/five-ants-watch.md

examples/agent/openai.ts

examples/agent/react_agent.ts

examples/agent/step_wise_query_tool.ts

examples/agent/stream_openai_agent.ts

examples/agent/wiki.ts

...es/core/e2e/examples/cloudflare-worker-agent/src/index.ts

...ages/core/e2e/examples/nextjs-agent/src/actions/index.tsx

packages/core/e2e/node/claude.e2e.ts

packages/core/e2e/node/openai.e2e.ts

packages/core/e2e/node/react.e2e.ts

packages/core/src/EngineResponse.ts

packages/core/src/Response.ts

packages/core/src/agent/anthropic.ts

packages/core/src/agent/base.ts

packages/core/src/engines/chat/CondenseQuestionChatEngine.ts

packages/core/src/engines/chat/ContextChatEngine.ts

packages/core/src/engines/chat/SimpleChatEngine.ts

packages/core/src/engines/chat/types.ts

packages/core/src/engines/query/RetrieverQueryEngine.ts