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/model-provider/src/auth.rs

@@ -412,7 +412,7 @@ mod tests {
             /*forced_chatgpt_workspace_id*/ None,
             /*chatgpt_base_url*/ None,
             AuthKeyringBackendKind::default(),
-            /*auth_route_config*/ None,
+            codex_login::test_support::transport_default_auth_route_config(),
         )
         .await;
         let auth = auth_manager.auth().await.expect("auth should load");
@@ -497,7 +497,7 @@ mod tests {
                 /*forced_chatgpt_workspace_id*/ None,
                 /*chatgpt_base_url*/ None,
                 AuthKeyringBackendKind::default(),
-                /*auth_route_config*/ None,
+                codex_login::test_support::transport_default_auth_route_config(),
             )
             .await,
         );