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/edit.rs

@@ -37,6 +37,8 @@ pub enum ConfigEdit {
     SetNoticeHideFullAccessWarning(bool),
     /// Toggle the Windows world-writable directories warning acknowledgement flag.
     SetNoticeHideWorldWritableWarning(bool),
+    /// Toggle the opt-out marker for Codex-managed fast defaults.
+    SetNoticeFastDefaultOptOut(bool),
     /// Toggle the rate limit model nudge acknowledgement flag.
     SetNoticeHideRateLimitModelNudge(bool),
     /// Toggle the Windows onboarding acknowledgement flag.
@@ -436,6 +438,11 @@ impl ConfigDocument {
                 &[NOTICE_TABLE_KEY, "hide_world_writable_warning"],
                 value(*acknowledged),
             )),
+            ConfigEdit::SetNoticeFastDefaultOptOut(opted_out) => Ok(self.write_value(
+                Scope::Global,
+                &[NOTICE_TABLE_KEY, "fast_default_opt_out"],
+                value(*opted_out),
+            )),
             ConfigEdit::SetNoticeHideRateLimitModelNudge(acknowledged) => Ok(self.write_value(
                 Scope::Global,
                 &[NOTICE_TABLE_KEY, "hide_rate_limit_model_nudge"],
@@ -978,6 +985,12 @@ impl ConfigEditsBuilder {
         self
     }
 
+    pub fn set_fast_default_opt_out(mut self, opted_out: bool) -> Self {
+        self.edits
+            .push(ConfigEdit::SetNoticeFastDefaultOptOut(opted_out));
+        self
+    }
+
     pub fn set_hide_rate_limit_model_nudge(mut self, acknowledged: bool) -> Self {
         self.edits
             .push(ConfigEdit::SetNoticeHideRateLimitModelNudge(acknowledged));