core: add wait_for_environment for starting environments

[sayan-oai]

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