Integrate Arcjet Guard into Python Google ADK

arcjet.guard.google_adk 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 Google ADK-specific.

Official Python google-adk>=2.0.0,<3 only — not JS @google/adk (@arcjet/guard/google-adk/v2, docs https://docs.arcjet.com/guards/google-adk/), not @google/genai, not Go / Java ADK. Importing arcjet.guard.google_adk does not load LangChain, CrewAI, or the JS adapter. The extra is safe (no chromadb).

Exports: guard_tool, guard_plugin, google_adk_context. There is no inbound helper and no approval helper.

Three surfaces, one decision rule: