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/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
     }