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/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(),
                 ),