Default Fast service tier for eligible ChatGPT plans (#19053)

## Why

Enterprise and business-like ChatGPT plans should get Codex's Fast
service tier by default when the user or caller has not made an explicit
service-tier choice. At the same time, callers need a durable way to
choose standard routing without adding a new persisted `standard`
service tier value. This keeps existing config compatibility while
letting core own the managed default policy.

## What changed

- Resolve the effective service tier in core at session creation:
explicit `fast` or `flex` wins, explicit null/clear or
`[notice].fast_default_opt_out = true` resolves to standard routing, and
otherwise eligible ChatGPT plans resolve to Fast when FastMode is
enabled.
- Add `[notice].fast_default_opt_out` as the persisted opt-out marker
for managed Fast defaults.
- Treat app-server/TUI `service_tier: null` as an explicit
standard/clear choice by preserving that intent through config loading.
- Update TUI rendering to use core's effective service tier for startup
and status surfaces while still keeping `config.service_tier` as the
explicit configured choice.
- Update `/fast off` to clear `service_tier`, persist the opt-out
marker, and send explicit standard for subsequent turns.

## Verification

- Added unit coverage for config override/notice handling, service-tier
resolution, runtime null clearing, and `/fast off` turn propagation.
- `cargo build -p codex-cli`

Full test suite was not run locally per author request.

codex-rs/core/src/session/tests.rs

@@ -26,6 +26,8 @@ use codex_models_manager::bundled_models_response;
 use codex_models_manager::model_info;
 use codex_protocol::AgentPath;
 use codex_protocol::ThreadId;
+use codex_protocol::account::PlanType as AccountPlanType;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::TrustLevel;
 use codex_protocol::exec_output::ExecToolCallOutput;
 use codex_protocol::models::FileSystemPermissions;
@@ -2643,6 +2645,104 @@ fn session_telemetry(
     )
 }
 
+#[test]
+fn get_service_tier_defaults_enterprise_accounts_to_fast() {
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ false,
+            Some(AccountPlanType::Enterprise),
+            /*fast_mode_enabled*/ true,
+        ),
+        Some(ServiceTier::Fast)
+    );
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ false,
+            Some(AccountPlanType::EnterpriseCbpUsageBased),
+            /*fast_mode_enabled*/ true,
+        ),
+        Some(ServiceTier::Fast)
+    );
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ false,
+            Some(AccountPlanType::Business),
+            /*fast_mode_enabled*/ true,
+        ),
+        Some(ServiceTier::Fast)
+    );
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ false,
+            Some(AccountPlanType::Team),
+            /*fast_mode_enabled*/ true,
+        ),
+        Some(ServiceTier::Fast)
+    );
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ false,
+            Some(AccountPlanType::SelfServeBusinessUsageBased),
+            /*fast_mode_enabled*/ true,
+        ),
+        Some(ServiceTier::Fast)
+    );
+}
+
+#[test]
+fn get_service_tier_respects_fast_default_opt_out() {
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ true,
+            Some(AccountPlanType::Enterprise),
+            /*fast_mode_enabled*/ true,
+        ),
+        None
+    );
+}
+
+#[test]
+fn get_service_tier_does_not_default_non_enterprise_or_disabled_fast_mode() {
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ false,
+            Some(AccountPlanType::Pro),
+            /*fast_mode_enabled*/ true,
+        ),
+        None
+    );
+    assert_eq!(
+        get_service_tier(
+            /*configured_service_tier*/ None,
+            /*fast_default_opt_out*/ false,
+            Some(AccountPlanType::Enterprise),
+            /*fast_mode_enabled*/ false,
+        ),
+        None
+    );
+}
+
+#[tokio::test]
+async fn session_settings_null_service_tier_update_clears_service_tier() {
+    let session_configuration = make_session_configuration_for_tests().await;
+
+    let updated = session_configuration
+        .apply(&SessionSettingsUpdate {
+            service_tier: Some(None),
+            ..Default::default()
+        })
+        .expect("null service tier update should apply");
+
+    assert_eq!(updated.service_tier, None);
+}
+
 pub(crate) async fn make_session_configuration_for_tests() -> SessionConfiguration {
     let codex_home = tempfile::tempdir().expect("create temp dir");
     let config = build_test_config(codex_home.path()).await;