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/config/mod.rs

@@ -2031,14 +2031,24 @@ impl Config {
         let forced_login_method = cfg.forced_login_method;
 
         let model = model.or(config_profile.model).or(cfg.model);
-        let service_tier = service_tier_override
-            .unwrap_or_else(|| config_profile.service_tier.or(cfg.service_tier));
+        let mut notices = cfg.notice.unwrap_or_default();
+        let service_tier = match service_tier_override {
+            Some(Some(service_tier)) => Some(service_tier),
+            Some(None) => {
+                // Preserve explicit standard/clear intent after the nested override
+                // collapses into `Config.service_tier = None`.
+                notices.fast_default_opt_out = Some(true);
+                None
+            }
+            None => config_profile.service_tier.or(cfg.service_tier),
+        };
         let service_tier = match service_tier {
             Some(ServiceTier::Fast) if features.enabled(Feature::FastMode) => {
                 Some(ServiceTier::Fast)
             }
+            Some(ServiceTier::Fast) => None,
             Some(ServiceTier::Flex) => Some(ServiceTier::Flex),
-            _ => None,
+            None => None,
         };
 
         let compact_prompt = compact_prompt.or(cfg.compact_prompt).and_then(|value| {
@@ -2414,7 +2424,7 @@ impl Config {
             active_profile: active_profile_name,
             active_project,
             windows_wsl_setup_acknowledged: cfg.windows_wsl_setup_acknowledged.unwrap_or(false),
-            notices: cfg.notice.unwrap_or_default(),
+            notices,
             check_for_update_on_startup,
             disable_paste_burst: cfg.disable_paste_burst.unwrap_or(false),
             analytics_enabled: config_profile