2- Use string service tiers in session protocol (#20971)

## Summary
- break service tier session/op/app-server protocol fields from the
closed enum to string tier ids
- send the service tier string directly through model requests, prewarm,
compaction, memories, and TUI/app-server turn starts
- regenerate app-server protocol JSON/TypeScript schemas, removing the
standalone ServiceTier TS enum

## Verification
- just fmt
- cargo check -p codex-core -p codex-app-server -p codex-tui
- just write-app-server-schema

---------

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

codex-rs/core/src/config/config_tests.rs

@@ -62,6 +62,7 @@ use codex_model_provider_info::LMSTUDIO_OSS_PROVIDER_ID;
 use codex_model_provider_info::OLLAMA_OSS_PROVIDER_ID;
 use codex_model_provider_info::WireApi;
 use codex_models_manager::bundled_models_response;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::models::ActivePermissionProfile;
 use codex_protocol::models::ActivePermissionProfileModification;
 use codex_protocol::models::ManagedFileSystemPermissions;
@@ -7132,6 +7133,28 @@ async fn explicit_null_service_tier_override_sets_fast_default_opt_out() -> std:
     Ok(())
 }
 
+#[tokio::test]
+async fn legacy_fast_service_tier_override_uses_priority_request_value() -> std::io::Result<()> {
+    let fixture = create_test_fixture()?;
+
+    let config = Config::load_from_base_config_with_overrides(
+        fixture.cfg.clone(),
+        ConfigOverrides {
+            cwd: Some(fixture.cwd_path()),
+            service_tier: Some(Some("fast".to_string())),
+            ..Default::default()
+        },
+        fixture.codex_home(),
+    )
+    .await?;
+
+    assert_eq!(
+        config.service_tier,
+        Some(ServiceTier::Fast.request_value().to_string())
+    );
+    Ok(())
+}
+
 #[tokio::test]
 async fn fast_default_opt_out_notice_config_is_respected() -> std::io::Result<()> {
     let fixture = create_test_fixture()?;