Allow hosts to customize wait_for_environment descriptions (#35106)

## What changed

- Add `WaitForEnvironmentToolConfig` as thread extension data for overriding the
  model-visible tool and `environment_id` descriptions.
- Preserve the default descriptions when no override is provided or when the
  configured descriptions exceed the input or serialized tool-spec limits.
- Keep `wait_for_environment` availability gated by the deferred executor
  feature independently of whether a host override is present.

## Testing

- Cover default, custom, oversized, and feature-disabled tool configurations.
- Verify the custom descriptions in the deferred-environment integration flow.

GitOrigin-RevId: 6b49a73a434becd99ea5df911be53f3706a17c0a

codex-rs/core-api/src/lib.rs

@@ -35,6 +35,7 @@ pub use codex_core::StartThreadOptions;
 pub use codex_core::StateDbHandle;
 pub use codex_core::ThreadManager;
 pub use codex_core::ThreadShutdownReport;
+pub use codex_core::WaitForEnvironmentToolConfig;
 pub use codex_core::build_models_manager;
 pub use codex_core::config::Config;
 pub use codex_core::config::Constrained;