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.rs

@@ -174,7 +174,7 @@ async fn run_remote_compact_task_inner_impl(
             CompactConversationRequestSettings {
                 effort: turn_context.reasoning_effort,
                 summary: turn_context.reasoning_summary,
-                service_tier: turn_context.config.service_tier,
+                service_tier: turn_context.config.service_tier.clone(),
             },
             &turn_context.session_telemetry,
             &compaction_trace,