Adds tests/e2e/codex: a harness that runs codex exec --json with a throwaway HOME and CODEX_HOME, a custom responses provider pointed at the gateway, the model list read from the gateway's /v1/models, the sandbox on and approvals off. Each test registers its setup's deployment via /model/new and calls it with a key scoped to it, both deleted on teardown. The first row checks that a value only a command can print comes back through the gateway on all five setups
