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/app-server-protocol/src/protocol/v2/config.rs

@@ -5,7 +5,6 @@ use super::shared::default_enabled;
 use codex_experimental_api_macros::ExperimentalApi;
 use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::ReasoningSummary;
-use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::config_types::WebSearchToolConfig;
@@ -137,7 +136,7 @@ pub struct ProfileV2 {
     /// used.
     #[experimental("config/read.approvalsReviewer")]
     pub approvals_reviewer: Option<ApprovalsReviewer>,
-    pub service_tier: Option<ServiceTier>,
+    pub service_tier: Option<String>,
     pub model_reasoning_effort: Option<ReasoningEffort>,
     pub model_reasoning_summary: Option<ReasoningSummary>,
     pub model_verbosity: Option<Verbosity>,
@@ -248,7 +247,7 @@ pub struct Config {
     pub model_reasoning_effort: Option<ReasoningEffort>,
     pub model_reasoning_summary: Option<ReasoningSummary>,
     pub model_verbosity: Option<Verbosity>,
-    pub service_tier: Option<ServiceTier>,
+    pub service_tier: Option<String>,
     pub analytics: Option<AnalyticsConfig>,
     #[experimental("config/read.apps")]
     #[serde(default)]