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/app-server/src/request_processors/thread_processor.rs

@@ -1188,7 +1188,7 @@ impl ThreadRequestProcessor {
         &self,
         model: Option<String>,
         model_provider: Option<String>,
-        service_tier: Option<Option<codex_protocol::config_types::ServiceTier>>,
+        service_tier: Option<Option<String>>,
         cwd: Option<String>,
         approval_policy: Option<codex_app_server_protocol::AskForApproval>,
         approvals_reviewer: Option<codex_app_server_protocol::ApprovalsReviewer>,

codex-rs/app-server/src/request_processors/thread_processor_tests.rs

@@ -629,7 +629,7 @@ mod thread_processor_behavior_tests {
             path: None,
             model: None,
             model_provider: None,
-            service_tier: Some(Some(codex_protocol::config_types::ServiceTier::Fast)),
+            service_tier: Some(Some("priority".to_string())),
             cwd: None,
             approval_policy: None,
             approvals_reviewer: None,
@@ -645,7 +645,7 @@ mod thread_processor_behavior_tests {
         let config_snapshot = ThreadConfigSnapshot {
             model: "gpt-5".to_string(),
             model_provider_id: "openai".to_string(),
-            service_tier: Some(codex_protocol::config_types::ServiceTier::Flex),
+            service_tier: Some("flex".to_string()),
             approval_policy: codex_protocol::protocol::AskForApproval::OnRequest,
             approvals_reviewer: codex_protocol::config_types::ApprovalsReviewer::User,
             permission_profile: codex_protocol::models::PermissionProfile::Disabled,
@@ -660,7 +660,7 @@ mod thread_processor_behavior_tests {
 
         assert_eq!(
             collect_resume_override_mismatches(&request, &config_snapshot),
-            vec!["service_tier requested=Some(Fast) active=Some(Flex)".to_string()]
+            vec!["service_tier requested=Some(\"priority\") active=Some(\"flex\")".to_string()]
         );
     }
 

codex-rs/app-server/src/request_processors/turn_processor.rs

@@ -438,7 +438,7 @@ impl TurnRequestProcessor {
                     model: model.clone(),
                     effort,
                     summary,
-                    service_tier,
+                    service_tier: service_tier.clone(),
                     collaboration_mode: collaboration_mode.clone(),
                     personality,
                 })