refactor: chat message type (#701)

examples/toolsStream.ts

packages/core/e2e/fixtures/llm/open_ai.ts

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

packages/core/src/llm/LLM.ts

packages/core/src/llm/anthropic.ts

packages/core/src/llm/base.ts

packages/core/src/llm/mistral.ts

packages/core/src/llm/open_ai.ts

packages/core/src/llm/types.ts

packages/core/src/llm/utils.ts

packages/core/src/memory/ChatMemoryBuffer.ts

packages/core/src/memory/types.ts

packages/core/src/storage/chatStore/SimpleChatStore.ts

packages/core/src/storage/chatStore/types.ts

packages/core/tests/utility/mockOpenAI.ts