Require auth managers to receive routing configuration (#34650)

## Why

Auth managers should use the application's resolved HTTP client factory instead
of silently falling back to the transport's default proxy behavior.

## What changed

- Make `AuthRouteConfig` required when constructing an `AuthManager` or
  `AuthConfig`.
- Pass each production caller's resolved routing configuration through without
  wrapping it in an optional value.
- Add a test helper that explicitly selects the transport-default proxy policy
  for callers that do not exercise custom routing.

GitOrigin-RevId: d89a3b1f8b5d4007650cdac0aae241c94d598580

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

@@ -1250,8 +1250,8 @@ impl AuthManagerConfig for Config {
         self.chatgpt_base_url.clone()
     }
 
-    fn auth_route_config(&self) -> Option<AuthRouteConfig> {
-        Some(Config::auth_route_config(self))
+    fn auth_route_config(&self) -> AuthRouteConfig {
+        Config::auth_route_config(self)
     }
 }