Coding agent suite tests

Use test/suite/ for the new harness-based test suite around AgentSession and AgentSessionRuntime.

Rules:

Use test/suite/harness.ts
Use the faux provider from packages/ai/src/providers/faux.ts
Do not use real provider APIs, real API keys, network calls, or paid tokens
Keep these tests CI-safe and deterministic
Do not use or extend the legacy test/test-harness.ts path unless a missing capability forces it