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/app-server/src/request_processors/turn_processor.rs

@@ -438,7 +438,7 @@ impl TurnRequestProcessor {
                     model: model.clone(),
                     effort,
                     summary,
-                    service_tier,
+                    service_tier: service_tier.clone(),
                     collaboration_mode: collaboration_mode.clone(),
                     personality,
                 })