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/src/session_startup_prewarm.rs

@@ -232,7 +232,7 @@ async fn schedule_startup_prewarm_inner(
             &startup_turn_context.session_telemetry,
             startup_turn_context.reasoning_effort,
             startup_turn_context.reasoning_summary,
-            startup_turn_context.config.service_tier,
+            startup_turn_context.config.service_tier.clone(),
             startup_turn_metadata_header.as_deref(),
         )
         .await?;