app-server service tier plumbing (plus some cleanup) (#13334)

followup to https://github.com/openai/codex/pull/13212 to expose fast
tier controls to app server
(majority of this PR is generated schema jsons - actual code is +69 /
-35 and +24 tests )

- add service tier fields to the app-server protocol surfaces used by
thread lifecycle, turn start, config, and session configured events
- thread service tier through the app-server message processor and core
thread config snapshots
- allow runtime config overrides to carry service tier for app-server
callers

cleanup:
- Removing useless "legacy" code supporting "standard" - we moved to
None | "fast", so "standard" is not needed.

codex-rs/protocol/src/protocol.rs

@@ -2783,6 +2783,9 @@ pub struct SessionConfiguredEvent {
 
     pub model_provider_id: String,
 
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub service_tier: Option<ServiceTier>,
+
     /// When to escalate for approval for execution
     pub approval_policy: AskForApproval,
 
@@ -3464,6 +3467,7 @@ mod tests {
                 thread_name: None,
                 model: "codex-mini-latest".to_string(),
                 model_provider_id: "openai".to_string(),
+                service_tier: None,
                 approval_policy: AskForApproval::Never,
                 sandbox_policy: SandboxPolicy::new_read_only_policy(),
                 cwd: PathBuf::from("/home/user/project"),