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

@@ -7,7 +7,6 @@ use codex_experimental_api_macros::ExperimentalApi;
 use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary;
-use codex_protocol::config_types::ServiceTier;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::plan_tool::PlanItemArg as CorePlanItemArg;
 use codex_protocol::plan_tool::StepStatus as CorePlanStepStatus;
@@ -94,7 +93,7 @@ pub struct TurnStartParams {
         skip_serializing_if = "Option::is_none"
     )]
     #[ts(optional = nullable)]
-    pub service_tier: Option<Option<ServiceTier>>,
+    pub service_tier: Option<Option<String>>,
     /// Override the reasoning effort for this turn and subsequent turns.
     #[ts(optional = nullable)]
     pub effort: Option<ReasoningEffort>,