fix: context not working in contextchatengine (#1237)

.changeset/cold-gifts-help.md

packages/core/src/memory/base.ts

packages/core/src/memory/chat-memory-buffer.ts

packages/core/src/memory/summary-memory.ts

packages/core/tests/memory/chat-memory-buffer.test.ts

packages/llamaindex/src/agent/base.ts

...llamaindex/src/engines/chat/CondenseQuestionChatEngine.ts

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

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