fix: enable resume_warning that was missing from mod.rs (#8333)

This test was introduced in https://github.com/openai/codex/pull/6507,
but was not included in `mod.rs`. It does not appear that it was getting
compiled?

codex-rs/core/tests/suite/resume_warning.rs

@@ -4,7 +4,6 @@ use codex_core::AuthManager;
 use codex_core::CodexAuth;
 use codex_core::ConversationManager;
 use codex_core::NewConversation;
-use codex_core::built_in_model_providers;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::InitialHistory;
 use codex_core::protocol::ResumedHistory;