feat: use json format for tool result (#742)

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

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

...re/e2e/node/snapshot/agent_with_object_function_call.snap

packages/core/e2e/node/snapshot/react-agent.snap

packages/core/src/agent/anthropic.ts

packages/core/src/agent/openai.ts

packages/core/src/agent/react.ts

packages/core/src/agent/utils.ts

packages/core/src/internal/utils.ts

packages/core/src/llm/open_ai.ts

packages/core/src/llm/types.ts

packages/core/src/tools/functionTool.ts

packages/core/src/types.ts