Add custom gateway selection to TUI sign-in (#60009)

## What changed

- Add the disabled-by-default `gateway_setup_wizard` feature to let users choose a saved custom gateway during eligible local sign-in onboarding.
- Save the selection as `model_provider`, restart the embedded app server with that provider, and complete required primary authentication and gateway OAuth before starting the first chat.
- Show gateway loading and configuration errors, browser sign-in links, and an OAuth retry action. Respect provider policy and launch overrides.

## Testing

Add coverage for onboarding eligibility, saved gateway selection and persistence, and gateway error and OAuth screens. Add a terminal integration test verifying that the selected gateway receives the first chat with authentication.

GitOrigin-RevId: 92bd2da3abd28d6c2274c91b2d7561bf359f0ca0

codex-rs/features/src/lib.rs

@@ -477,6 +477,8 @@ pub enum Feature {
     BedrockSetupWizard,
     /// Offer Google Cloud Vertex AI setup during TUI sign-in onboarding.
     GcpSetupWizard,
+    /// Offer custom gateway setup during TUI sign-in onboarding.
+    GatewaySetupWizard,
     /// Removed compatibility flag retained as a no-op.
     Personality,
     /// Enable native artifact tools.
@@ -2262,6 +2264,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::GatewaySetupWizard,
+        key: "gateway_setup_wizard",
+        stage: Stage::Stable,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::Personality,
         key: "personality",