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/tui/src/app/event_dispatch.rs

@@ -1263,7 +1263,8 @@ impl App {
             AppEvent::PersistServiceTierSelection { service_tier } => {
                 self.refresh_status_line();
                 let profile = self.active_profile.as_deref();
-                self.config.service_tier = service_tier;
+                self.config.service_tier =
+                    service_tier.map(|service_tier| service_tier.request_value().to_string());
                 let mut edits = ConfigEditsBuilder::new(&self.config.codex_home)
                     .with_profile(profile)
                     .set_service_tier(service_tier);

codex-rs/tui/src/app/session_lifecycle.rs

@@ -617,7 +617,10 @@ impl App {
 
     pub(super) fn fresh_session_config(&self) -> Config {
         let mut config = self.config.clone();
-        config.service_tier = self.chat_widget.configured_service_tier();
+        config.service_tier = self
+            .chat_widget
+            .configured_service_tier()
+            .map(|service_tier| service_tier.request_value().to_string());
         config.notices.fast_default_opt_out = self.chat_widget.fast_default_opt_out();
         config
     }

codex-rs/tui/src/app/tests.rs

@@ -4496,7 +4496,11 @@ async fn fresh_session_config_uses_current_service_tier() {
 
     assert_eq!(
         config.service_tier,
-        Some(codex_protocol::config_types::ServiceTier::Fast)
+        Some(
+            codex_protocol::config_types::ServiceTier::Fast
+                .request_value()
+                .to_string()
+        )
     );
 }
 

codex-rs/tui/src/app/thread_routing.rs

@@ -623,7 +623,7 @@ impl App {
                             model.to_string(),
                             *effort,
                             *summary,
-                            *service_tier,
+                            service_tier.clone(),
                             collaboration_mode.clone(),
                             *personality,
                             final_output_json_schema.clone(),

codex-rs/tui/src/app/thread_session_state.rs

@@ -63,7 +63,10 @@ impl App {
                 thread_name: None,
                 model: self.chat_widget.current_model().to_string(),
                 model_provider_id: self.config.model_provider_id.clone(),
-                service_tier: self.chat_widget.current_service_tier(),
+                service_tier: self
+                    .chat_widget
+                    .current_service_tier()
+                    .map(|service_tier| service_tier.request_value().to_string()),
                 approval_policy: AskForApproval::from(
                     self.config.permissions.approval_policy.value(),
                 ),