[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/app-server/tests/suite/v2/turn_start.rs

@@ -54,6 +54,7 @@ use codex_app_server_protocol::UserInput as V2UserInput;
 use codex_app_server_protocol::WarningNotification;
 use codex_config::config_toml::ConfigToml;
 use codex_core::personality_migration::PERSONALITY_MIGRATION_FILENAME;
+use codex_core::test_support::all_model_presets;
 use codex_features::FEATURES;
 use codex_features::Feature;
 use codex_protocol::config_types::CollaborationMode;
@@ -375,13 +376,19 @@ async fn turn_start_sends_service_tier_id_to_model_request() -> Result<()> {
         "never",
         &BTreeMap::default(),
     )?;
+    write_models_cache(codex_home.path())?;
+    let service_tier_model = all_model_presets()
+        .iter()
+        .find(|preset| preset.show_in_picker && !preset.service_tiers.is_empty())
+        .expect("bundled model catalog should include a picker model with service tiers");
+    let service_tier_id = service_tier_model.service_tiers[0].id.clone();
 
     let mut mcp = McpProcess::new(codex_home.path()).await?;
     timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
 
     let thread_req = mcp
         .send_thread_start_request(ThreadStartParams {
-            model: Some("mock-model".to_string()),
+            model: Some(service_tier_model.id.clone()),
             ..Default::default()
         })
         .await?;
@@ -392,7 +399,6 @@ async fn turn_start_sends_service_tier_id_to_model_request() -> Result<()> {
     .await??;
     let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(thread_resp)?;
 
-    let service_tier_id = "experimental-tier-id".to_string();
     let turn_req = mcp
         .send_turn_start_request(TurnStartParams {
             thread_id: thread.id,