Add optional dependency groups for framework adapters

[mertsatilmaz]

Goal

Add optional dependency groups for future framework adapters.

The base package should remain lightweight and vendor-neutral. Framework dependencies should be opt-in.

Why this matters

Users should be able to install and use the core harness without pulling OpenAI Agents SDK, LangChain, LangGraph, or MCP dependencies.

Framework adapters should use optional extras.

Suggested optional extras

Add placeholder or initial optional dependency groups in pyproject.toml, such as:

[project.optional-dependencies]
openai-agents = []
langchain = []
mcp = []