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/core/tests/suite/agent_websocket.rs

@@ -313,7 +313,7 @@ async fn websocket_v2_first_turn_drops_fast_tier_after_startup_prewarm() -> Resu
             .features
             .enable(Feature::ResponsesWebsocketsV2)
             .expect("test config should allow feature update");
-        config.service_tier = Some(ServiceTier::Fast);
+        config.service_tier = Some(ServiceTier::Fast.request_value().to_string());
     });
     let test = builder.build_with_websocket_server(&server).await?;