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/exec/src/lib.rs

@@ -1066,7 +1066,7 @@ fn session_configured_from_thread_start_response(
         response.thread.path.clone(),
         response.model.clone(),
         response.model_provider.clone(),
-        response.service_tier,
+        response.service_tier.clone(),
         response.approval_policy.to_core(),
         response.approvals_reviewer.to_core(),
         response
@@ -1091,7 +1091,7 @@ fn session_configured_from_thread_resume_response(
         response.thread.path.clone(),
         response.model.clone(),
         response.model_provider.clone(),
-        response.service_tier,
+        response.service_tier.clone(),
         response.approval_policy.to_core(),
         response.approvals_reviewer.to_core(),
         response
@@ -1125,7 +1125,7 @@ fn session_configured_from_thread_response(
     rollout_path: Option<PathBuf>,
     model: String,
     model_provider_id: String,
-    service_tier: Option<codex_protocol::config_types::ServiceTier>,
+    service_tier: Option<String>,
     approval_policy: AskForApproval,
     approvals_reviewer: codex_protocol::config_types::ApprovalsReviewer,
     permission_profile: PermissionProfile,