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/app/event_dispatch.rs

@@ -1029,14 +1029,24 @@ impl App {
             AppEvent::PersistServiceTierSelection { service_tier } => {
                 self.refresh_status_line();
                 let profile = self.active_profile.as_deref();
-                match ConfigEditsBuilder::new(&self.config.codex_home)
+                self.config.service_tier = service_tier;
+                let mut edits = ConfigEditsBuilder::new(&self.config.codex_home)
                     .with_profile(profile)
-                    .set_service_tier(service_tier)
-                    .apply()
-                    .await
-                {
+                    .set_service_tier(service_tier);
+                if service_tier.is_none() {
+                    self.config.notices.fast_default_opt_out = Some(true);
+                    edits = edits.set_fast_default_opt_out(/*opted_out*/ true);
+                }
+                match edits.apply().await {
                     Ok(()) => {
-                        let status = if service_tier.is_some() { "on" } else { "off" };
+                        let status = if matches!(
+                            service_tier,
+                            Some(codex_protocol::config_types::ServiceTier::Fast)
+                        ) {
+                            "on"
+                        } else {
+                            "off"
+                        };
                         let mut message = format!("Fast mode set to {status}");
                         if let Some(profile) = profile {
                             message.push_str(" for ");