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_routing.rs

@@ -623,7 +623,7 @@ impl App {
                             model.to_string(),
                             *effort,
                             *summary,
-                            *service_tier,
+                            service_tier.clone(),
                             collaboration_mode.clone(),
                             *personality,
                             final_output_json_schema.clone(),