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

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Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/tests/suite/compact_remote.rs

@@ -445,7 +445,7 @@ async fn assert_remote_manual_compact_request_parity(
     let mut builder = test_codex().with_auth(auth);
     if let Some(service_tier) = configured_service_tier {
         builder = builder.with_config(move |config| {
-            config.service_tier = Some(service_tier);
+            config.service_tier = Some(service_tier.request_value().to_string());
         });
     }
     let harness = TestCodexHarness::with_builder(builder).await?;