refactor: use `event.reason`, remove `parentEvent` (#681)

examples/chatHistory.ts

packages/core/src/GlobalsHelper.ts

packages/core/src/Retriever.ts

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

packages/core/src/callbacks/CallbackManager.ts

packages/core/src/cloud/LlamaCloudRetriever.ts

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

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

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

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

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

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

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

packages/core/src/indices/summary/index.ts

packages/core/src/indices/vectorStore/index.ts

packages/core/src/internal/context/EventCaller.ts

packages/core/src/internal/utils.ts

packages/core/src/llm/LLM.ts

packages/core/src/llm/base.ts

packages/core/src/llm/mistral.ts