[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/model-provider/src/amazon_bedrock/catalog.rs

@@ -1,14 +1,17 @@
 use codex_models_manager::model_info::BASE_INSTRUCTIONS;
 use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::openai_models::ApplyPatchToolType;
 use codex_protocol::openai_models::ConfigShellToolType;
 use codex_protocol::openai_models::InputModality;
 use codex_protocol::openai_models::ModelInfo;
+use codex_protocol::openai_models::ModelServiceTier;
 use codex_protocol::openai_models::ModelVisibility;
 use codex_protocol::openai_models::ModelsResponse;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::openai_models::ReasoningEffortPreset;
+use codex_protocol::openai_models::SPEED_TIER_FAST;
 use codex_protocol::openai_models::TruncationPolicyConfig;
 use codex_protocol::openai_models::WebSearchToolType;
 
@@ -46,8 +49,12 @@ fn gpt_5_4_cmb_bedrock_model(priority: i32) -> ModelInfo {
         visibility: ModelVisibility::List,
         supported_in_api: true,
         priority,
-        additional_speed_tiers: vec!["fast".to_string()],
-        service_tiers: Vec::new(),
+        additional_speed_tiers: Vec::new(),
+        service_tiers: vec![ModelServiceTier {
+            id: ServiceTier::Fast.request_value().to_string(),
+            name: SPEED_TIER_FAST.to_string(),
+            description: "Fastest inference with increased plan usage".to_string(),
+        }],
         availability_nux: None,
         upgrade: None,
         base_instructions: BASE_INSTRUCTIONS.to_string(),