1- Add model service tiers metadata (#20969)

## Why

The model list needs to carry display-ready service tier metadata so
clients can render tier choices with stable IDs, names, and
descriptions. A raw speed-tier string list is not enough for richer UI
copy or future tier labels.

## What changed

- Added `ModelServiceTier` to shared model metadata with string `id`,
`name`, and `description` fields.
- Added `service_tiers` to `ModelInfo` and `ModelPreset`, preserving
empty defaults for older cached model payloads.
- Exposed `serviceTiers` on app-server v2 `Model` responses and threaded
it through TUI app-server model conversion.
- Marked legacy `additional_speed_tiers` / `additionalSpeedTiers`
metadata as deprecated in source and generated schema output.
- Regenerated app-server protocol JSON schema and TypeScript fixtures,
including `ModelServiceTier.ts`.

## Verification

- Ran `just write-app-server-schema`.
- Did not run local tests per repo instruction; relying on PR CI.

---------

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

codex-rs/codex-api/tests/models_integration.rs

@@ -75,6 +75,7 @@ async fn models_client_hits_models_endpoint() {
             supported_in_api: true,
             priority: 1,
             additional_speed_tiers: Vec::new(),
+            service_tiers: Vec::new(),
             upgrade: None,
             base_instructions: "base instructions".to_string(),
             model_messages: None,