feat: allow `tool_choice` for OpenAIAgent (#1472)

packages/core/src/agent/base.ts

packages/core/src/agent/llm.ts

packages/core/src/agent/types.ts

packages/core/src/chat-engine/base.ts

packages/providers/anthropic/src/agent.ts

packages/providers/ollama/src/agent.ts

packages/providers/openai/package.json

packages/providers/openai/src/agent.ts

pnpm-lock.yaml