Enable conversation-scoped speed selection for Cloud turns (#61178)

## What changed

- Stage Cloud speed choices for the next new turn in the conversation, alongside pending model and effort selections. Update the displayed conversation speed after admission without changing the local default.
- Offer speed commands supported by the pending model. Clear an incompatible pending speed change with a warning when switching models.
- Carry `service_tier` through Cloud message requests, preserving omission to inherit the thread tier and explicit null to clear it. Translate `default` to null and reject tier overrides on queued requests.

## Testing

Add tests for Cloud speed toggling, model-dependent speed choices, admission-time updates, and preserving the conversation tier when selecting a model. Extend request and wire tests to cover service-tier values, explicit clearing, and queue rejection.

GitOrigin-RevId: b4daa701f1fe29593d90a938e60cc20cb980298a

codex-rs/tui/src/chatwidget.rs

@@ -554,6 +554,8 @@ pub(crate) struct ChatWidget {
     model_popup_request_id: Option<uuid::Uuid>,
     pub(crate) cloud_picker_selection:
         Option<crate::app_server_session::user_message_submission::CloudModelSelection>,
+    pub(crate) pending_cloud_turn_settings:
+        Option<crate::app_server_session::user_message_submission::PendingCloudTurnSettings>,
     /// An app event has been queued to open the permissions picker, but no view exists yet.
     pub(crate) permission_popup_pending: bool,
     permission_popup_request_id: Option<uuid::Uuid>,
@@ -570,6 +572,7 @@ pub(crate) struct ChatWidget {
     pub(crate) remote_connection: Option<RemoteConnectionStatus>,
     /// Remote app servers cannot read image paths on the TUI host.
     pub(crate) snapshot_local_images: bool,
+    pub(crate) thread_settings_owner: crate::app_server_session::ThreadSettingsOwner,
     pending_image_submission: Option<image_submission::PendingImageSubmission>,
     pub(crate) local_worktree_operations: bool,
     pub(crate) windows_sandbox_local_server: bool,