Validates forced plugin harness support for the requested provider/model before pinning Codex or any other plugin harness. This prevents an explicitly forced Codex runtime from accepting unsupported OpenAI-like providers through a hardcoded bypass while preserving implicit PI fallback and CLI runtime alias passthrough.
Regression coverage covers forced Codex rejection for unsupported openai/openai-codex support, Codex provider support declarations, CLI attempt routing, pi-embedded auth/profile forwarding fakes, Testbox scenario probes, and live Docker Codex plugin E2E.
Thanks @cathrynlavery.