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/compact_remote_v2.rs

@@ -252,7 +252,7 @@ async fn run_remote_compaction_request_v2(
             &turn_context.session_telemetry,
             turn_context.reasoning_effort,
             turn_context.reasoning_summary,
-            turn_context.config.service_tier,
+            turn_context.config.service_tier.clone(),
             turn_metadata_header,
             &InferenceTraceContext::disabled(),
         )