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/slash_dispatch.rs

@@ -172,7 +172,7 @@ impl ChatWidget {
                 self.open_model_popup();
             }
             SlashCommand::Fast => {
-                let next_tier = if matches!(self.config.service_tier, Some(ServiceTier::Fast)) {
+                let next_tier = if matches!(self.current_service_tier(), Some(ServiceTier::Fast)) {
                     None
                 } else {
                     Some(ServiceTier::Fast)
@@ -527,12 +527,12 @@ impl ChatWidget {
                     "on" => self.set_service_tier_selection(Some(ServiceTier::Fast)),
                     "off" => self.set_service_tier_selection(/*service_tier*/ None),
                     "status" => {
-                        let status = if matches!(self.config.service_tier, Some(ServiceTier::Fast))
-                        {
-                            "on"
-                        } else {
-                            "off"
-                        };
+                        let status =
+                            if matches!(self.current_service_tier(), Some(ServiceTier::Fast)) {
+                                "on"
+                            } else {
+                                "off"
+                            };
                         self.add_info_message(
                             format!("Fast mode is {status}."),
                             /*hint*/ None,