[codex] Generalize service tier slash commands (#21745)

## Why

`/fast` was wired as a one-off slash command even though model metadata
now exposes service tiers as catalog data. That meant adding another
tier, such as a slower/cheaper tier, would require more hardcoded TUI
plumbing instead of letting the model catalog drive the available
commands.

This change makes service-tier commands data-driven: each advertised
`service_tiers` entry becomes a `/name` command using the catalog
description, while the request path sends the tier `id` only when the
selected model supports it.

## What Changed

- Removed the hardcoded `/fast` slash-command variant and introduced
dynamic service-tier command items in the composer and command popup.
- Added toggle behavior for service-tier commands: invoking `/name`
selects that tier, and invoking it again clears the selection.
- Preserved the existing Fast-mode keybinding/status affordances by
resolving the current model tier whose name is `fast`, while still
sending the tier request value such as `priority`.
- Persisted service-tier selections as raw request strings so non-fast
tiers can round-trip through config.
- Updated the Bedrock catalog entry to advertise fast support through
`service_tiers` with `id: "priority"` and `name: "fast"`.
- Added defensive filtering in core so unsupported selected service
tiers are omitted from `/responses` requests.

## Validation

- Added/updated coverage for dynamic service-tier slash command lookup,
popup descriptions, composer dispatch, TUI fast toggling, and
unsupported-tier omission in core request construction.
- Local tests were not run per request.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/openai_models.rs

@@ -17,6 +17,7 @@ use ts_rs::TS;
 
 use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary;
+use crate::config_types::ServiceTier;
 use crate::config_types::Verbosity;
 
 const PERSONALITY_PLACEHOLDER: &str = "{{ personality }}";
@@ -479,13 +480,23 @@ impl ModelPreset {
     pub fn supports_fast_mode(&self) -> bool {
         self.service_tiers
             .iter()
-            .any(|tier| tier.id == SPEED_TIER_FAST)
+            .any(|tier| tier.id == ServiceTier::Fast.request_value())
             || self
                 .additional_speed_tiers
                 .iter()
                 .any(|tier| tier == SPEED_TIER_FAST)
     }
+}
 
+impl ModelInfo {
+    pub fn supports_service_tier(&self, service_tier: &str) -> bool {
+        self.service_tiers
+            .iter()
+            .any(|tier| tier.id == service_tier)
+    }
+}
+
+impl ModelPreset {
     /// Filter models based on authentication mode.
     ///
     /// In ChatGPT mode, all models are visible. Otherwise, only API-supported models are shown.
@@ -853,7 +864,7 @@ mod tests {
     fn model_preset_supports_fast_mode_from_service_tiers() {
         let preset = ModelPreset::from(ModelInfo {
             service_tiers: vec![ModelServiceTier {
-                id: SPEED_TIER_FAST.to_string(),
+                id: ServiceTier::Fast.request_value().to_string(),
                 name: "Fast".to_string(),
                 description: "Priority processing.".to_string(),
             }],