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/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()
+        )
     );
 }