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/tui/src/chatwidget.rs

@@ -776,6 +776,8 @@ pub(crate) struct ChatWidget {
     /// where the overlay may briefly treat new tail content as already cached.
     active_cell_revision: u64,
     config: Config,
+    /// Runtime value resolved by core. `config.service_tier` remains the explicit user choice.
+    effective_service_tier: Option<ServiceTier>,
     /// The unmasked collaboration mode settings (always Default mode).
     ///
     /// Masks are applied on top of this base mode to derive the effective mode.
@@ -2120,6 +2122,7 @@ impl ChatWidget {
         self.current_rollout_path = event.rollout_path.clone();
         self.current_cwd = Some(event.cwd.to_path_buf());
         self.config.cwd = event.cwd.clone();
+        self.effective_service_tier = event.service_tier;
         if let Err(err) = self
             .config
             .permissions
@@ -5106,6 +5109,7 @@ impl ChatWidget {
         let active_cell = Some(Self::placeholder_session_header_cell(&config));
 
         let current_cwd = Some(config.cwd.to_path_buf());
+        let effective_service_tier = config.service_tier;
         let queued_message_edit_binding = queued_message_edit_binding_for_terminal(terminal_info());
         let mut widget = Self {
             app_event_tx: app_event_tx.clone(),
@@ -5124,6 +5128,7 @@ impl ChatWidget {
             active_cell,
             active_cell_revision: 0,
             config,
+            effective_service_tier,
             skills_all: Vec::new(),
             skills_initial_state: None,
             current_collaboration_mode,
@@ -5942,7 +5947,11 @@ impl ChatWidget {
             .personality
             .filter(|_| self.config.features.enabled(Feature::Personality))
             .filter(|_| self.current_model_supports_personality());
-        let service_tier = Some(self.config.service_tier);
+        let service_tier = match self.config.service_tier {
+            Some(service_tier) => Some(Some(service_tier)),
+            None if self.config.notices.fast_default_opt_out == Some(true) => Some(None),
+            None => None,
+        };
         let op = AppCommand::user_turn(
             items,
             self.config.cwd.to_path_buf(),
@@ -9850,12 +9859,21 @@ impl ChatWidget {
     /// Set Fast mode in the widget's config copy.
     pub(crate) fn set_service_tier(&mut self, service_tier: Option<ServiceTier>) {
         self.config.service_tier = service_tier;
+        self.effective_service_tier = service_tier;
     }
 
     pub(crate) fn current_service_tier(&self) -> Option<ServiceTier> {
+        self.effective_service_tier
+    }
+
+    pub(crate) fn configured_service_tier(&self) -> Option<ServiceTier> {
         self.config.service_tier
     }
 
+    pub(crate) fn fast_default_opt_out(&self) -> Option<bool> {
+        self.config.notices.fast_default_opt_out
+    }
+
     pub(crate) fn status_account_display(&self) -> Option<&StatusAccountDisplay> {
         self.status_account_display.as_ref()
     }
@@ -9932,6 +9950,9 @@ impl ChatWidget {
     }
 
     fn set_service_tier_selection(&mut self, service_tier: Option<ServiceTier>) {
+        if service_tier.is_none() {
+            self.config.notices.fast_default_opt_out = Some(true);
+        }
         self.set_service_tier(service_tier);
         self.app_event_tx.send(AppEvent::CodexOp(
             AppCommand::override_turn_context(