Integrate Arcjet Guard into Python OpenAI Agents

arcjet.guard.openai_agents wraps the agent's existing Arcjet client. It never talks to the Arcjet API itself. Shared Guard fundamentals (client, rules, labels, decisions, capture, registration) live in ../arcjet/references/guards_python.md. Load that reference for anything that is not OpenAI Agents-specific.

Official openai-agents>=0.19.0,<1 only — not the JS @openai/agents adapter (@arcjet/guard/openai-agents/v0, docs https://docs.arcjet.com/guards/openai-agents/), not community forks. Importing arcjet.guard.openai_agents does not load LangChain.

Exports: guard_tool, openai_agents_context. Authored FunctionTool / @function_tool only. Not hosted tools, MCP, Computer / Shell / ApplyPatch, handoffs, or Agent.as_tool().

Two surfaces, one decision rule: