## Why
With `DeferredExecutor`, a sampling request can begin while an
environment is still starting. The model can see that pending state, but
needs a way to wait for the environment within the same turn before
continuing.
Environment startup is owned by Core, so the wait tool should use the
same request-frozen `StepContext` that advertised the starting
environment. This keeps tool registration and execution tied to the
exact startup operation the model saw, even if live thread state later
changes.
Supersedes #29735.
## What
- register `wait_for_environment` when the current `StepContext`
contains starting environments
- wait on the selected `StartingTurnEnvironment` shared resolution and
return a bounded ready or failed result
- rebuild the next request normally, removing the wait tool and exposing
ready environment tools, or reporting the environment as unavailable
after failure
## Testing
- `just test -p codex-core deferred_executor_`
- verifies the wait tool is replaced by environment-backed tools after
startup
- verifies startup failure removes both the wait tool and unavailable
environment tools while notifying the model
sayan-oai
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