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/config/src/profile_toml.rs

@@ -23,6 +23,7 @@ use codex_protocol::protocol::AskForApproval;
 #[schemars(deny_unknown_fields)]
 pub struct ConfigProfile {
     pub model: Option<String>,
+    /// Optional explicit service tier preference for new turns (`fast` or `flex`).
     pub service_tier: Option<ServiceTier>,
     /// The key in the `model_providers` map identifying the
     /// [`ModelProviderInfo`] to use.

codex-rs/config/src/types.rs

@@ -615,6 +615,8 @@ pub struct Notice {
     pub hide_full_access_warning: Option<bool>,
     /// Tracks whether the user has acknowledged the Windows world-writable directories warning.
     pub hide_world_writable_warning: Option<bool>,
+    /// Tracks whether the user opted out of Codex-managed fast defaults.
+    pub fast_default_opt_out: Option<bool>,
     /// Tracks whether the user opted out of the rate limit model switch reminder.
     pub hide_rate_limit_model_nudge: Option<bool>,
     /// Tracks whether the user has seen the model migration prompt