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/mod.rs

@@ -588,7 +588,7 @@ impl Codex {
             .auth_cached()
             .and_then(|auth| auth.account_plan_type());
         let service_tier = get_service_tier(
-            config.service_tier,
+            config.service_tier.clone(),
             config.notices.fast_default_opt_out.unwrap_or(false),
             account_plan_type,
             config.features.enabled(Feature::FastMode),
@@ -787,18 +787,18 @@ impl Codex {
 }
 
 fn get_service_tier(
-    configured_service_tier: Option<ServiceTier>,
+    configured_service_tier: Option<String>,
     fast_default_opt_out: bool,
     account_plan_type: Option<AccountPlanType>,
     fast_mode_enabled: bool,
-) -> Option<ServiceTier> {
+) -> Option<String> {
     if configured_service_tier.is_some() || fast_default_opt_out || !fast_mode_enabled {
         return configured_service_tier;
     }
 
     account_plan_type
         .is_some_and(is_enterprise_default_service_tier_plan)
-        .then_some(ServiceTier::Fast)
+        .then_some(ServiceTier::Fast.request_value().to_string())
 }
 
 fn is_enterprise_default_service_tier_plan(plan_type: AccountPlanType) -> bool {