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/config_lock.rs

@@ -109,7 +109,10 @@ fn save_session_resolved_fields(sc: &SessionConfiguration, lock_config: &mut Con
     lock_config.model = Some(sc.collaboration_mode.model().to_string());
     lock_config.model_reasoning_effort = sc.collaboration_mode.reasoning_effort();
     lock_config.model_reasoning_summary = sc.model_reasoning_summary;
-    lock_config.service_tier = sc.service_tier;
+    lock_config.service_tier = sc
+        .service_tier
+        .as_deref()
+        .and_then(codex_protocol::config_types::ServiceTier::from_request_value);
     lock_config.instructions = Some(sc.base_instructions.clone());
     lock_config.developer_instructions = sc.developer_instructions.clone();
     lock_config.compact_prompt = sc.compact_prompt.clone();
@@ -280,7 +283,7 @@ mod tests {
         sc.base_instructions = "catalog instructions".to_string();
         sc.developer_instructions = Some("catalog developer instructions".to_string());
         sc.compact_prompt = Some("catalog compact prompt".to_string());
-        sc.service_tier = Some(codex_protocol::config_types::ServiceTier::Flex);
+        sc.service_tier = Some("flex".to_string());
 
         let lockfile = sc.to_config_lockfile_toml().expect("lock should serialize");
         let lock = &lockfile.config;