[codex] Honor role-defined spawn service tiers (#22169)

## Why
Custom agent roles are ordinary config layers, so a role file can
already express `service_tier` just like other config values. The
spawned-agent tier path needs to preserve that effective role config and
follow the same precedence pattern as model/reasoning.

## What changed
- Apply an explicit spawn-time `service_tier` onto the child config
before role application, so a role config layer can override it just
like role-defined model/reasoning settings do.
- Validate the final effective child tier after the final child model is
known, while still falling back to the parent tier when no child tier
survives.
- Add focused integration coverage for both v1 and v2 proving role TOML
loads a service tier, spawned children keep that role-configured tier,
and a role tier wins over a conflicting spawn-time tier.

## Validation
- `just fmt`
- `git diff --check`
- Local Rust tests not run, per repo guidance; CI should exercise the
new coverage.

codex-rs/core/src/agent/role_tests.rs

@@ -6,6 +6,7 @@ use crate::skills_load_input_from_config;
 use codex_config::ConfigLayerStackOrdering;
 use codex_core_plugins::PluginsManager;
 use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_utils_absolute_path::test_support::PathExt;
@@ -214,6 +215,66 @@ async fn apply_role_preserves_unspecified_keys() {
     );
 }
 
+#[tokio::test]
+async fn apply_role_reports_explicit_service_tier() {
+    let (home, mut config) = test_config_with_cli_overrides(Vec::new()).await;
+    let role_path = write_role_config(
+        &home,
+        "tiered-role.toml",
+        r#"developer_instructions = "Stay focused"
+service_tier = "priority"
+"#,
+    )
+    .await;
+    config.agent_roles.insert(
+        "custom".to_string(),
+        AgentRoleConfig {
+            description: None,
+            config_file: Some(role_path),
+            nickname_candidates: None,
+        },
+    );
+
+    apply_role_to_config(&mut config, Some("custom"))
+        .await
+        .expect("custom role should apply");
+
+    assert_eq!(
+        config.service_tier,
+        Some(ServiceTier::Fast.request_value().to_string())
+    );
+}
+
+#[tokio::test]
+async fn apply_role_preserves_existing_service_tier_without_override() {
+    let (home, mut config) = test_config_with_cli_overrides(Vec::new()).await;
+    config.service_tier = Some(ServiceTier::Fast.request_value().to_string());
+    let role_path = write_role_config(
+        &home,
+        "default-tier-role.toml",
+        r#"developer_instructions = "Stay focused"
+"#,
+    )
+    .await;
+    config.agent_roles.insert(
+        "custom".to_string(),
+        AgentRoleConfig {
+            description: None,
+            config_file: Some(role_path),
+            nickname_candidates: None,
+        },
+    );
+
+    apply_role_to_config(&mut config, Some("custom"))
+        .await
+        .expect("custom role should apply");
+
+    assert_eq!(
+        config.service_tier,
+        Some(ServiceTier::Fast.request_value().to_string())
+    );
+}
+
 #[tokio::test]
 async fn apply_role_preserves_active_profile_and_model_provider() {
     let home = TempDir::new().expect("create temp dir");
@@ -766,6 +827,31 @@ fn spawn_tool_spec_marks_role_locked_reasoning_effort_only() {
         ));
 }
 
+#[test]
+fn spawn_tool_spec_marks_role_locked_service_tier() {
+    let tempdir = TempDir::new().expect("create temp dir");
+    let role_path = tempdir.path().join("tiered.toml");
+    fs::write(
+        &role_path,
+        "developer_instructions = \"Stay fast\"\nservice_tier = \"priority\"\n",
+    )
+    .expect("write role config");
+    let user_defined_roles = BTreeMap::from([(
+        "tiered".to_string(),
+        AgentRoleConfig {
+            description: Some("Stay fast.".to_string()),
+            config_file: Some(role_path),
+            nickname_candidates: None,
+        },
+    )]);
+
+    let spec = spawn_tool_spec::build(&user_defined_roles);
+
+    assert!(spec.contains(
+        "Stay fast.\n- This role's service tier is set to `priority`. If it is supported by the resolved model, it takes precedence over a valid spawn request service tier."
+    ));
+}
+
 #[test]
 fn built_in_config_file_contents_resolves_explorer_only() {
     assert_eq!(