[codex] Enable auth elicitation by default (#28772)

## Summary

- enable `auth_elicitation` by default
- promote the feature to `Stable`, as default-enabled features must be
stable
- update the feature regression test to cover the new lifecycle state
and default

## Impact

Auth elicitation is now available without requiring users or clients to
opt in through configuration.

## Testing

- `just test -p codex-features` (52 passed)

codex-rs/core/src/config/config_tests.rs

@@ -6039,13 +6039,6 @@ async fn to_mcp_config_preserves_auth_elicitation_feature_from_config() -> std::
     .await?;
     let plugins_manager = PluginsManager::new(codex_home.path().to_path_buf());
 
-    let mcp_config = config.to_mcp_config(&plugins_manager).await;
-    assert_eq!(
-        mcp_config.client_elicitation_capability,
-        ElicitationCapability::default()
-    );
-
-    let _ = config.features.enable(Feature::AuthElicitation);
     let mcp_config = config.to_mcp_config(&plugins_manager).await;
     assert_eq!(
         mcp_config.client_elicitation_capability,
@@ -6055,6 +6048,13 @@ async fn to_mcp_config_preserves_auth_elicitation_feature_from_config() -> std::
         }
     );
 
+    let _ = config.features.disable(Feature::AuthElicitation);
+    let mcp_config = config.to_mcp_config(&plugins_manager).await;
+    assert_eq!(
+        mcp_config.client_elicitation_capability,
+        ElicitationCapability::default()
+    );
+
     Ok(())
 }