[codex] Dedupe fallback model metadata warnings (#21090)

Fixes #21070.

This is a small cleanup around model metadata handling for
gateway/provider model names. It follows the report and proposed
direction from @dkbush by keeping the fallback metadata warning useful
without repeating it every turn, and by tightening the existing
provider-prefix lookup path.

- Track fallback metadata warning slugs in session state so each
unresolved model warns once per session.
- Keep warning emission outside the session-state lock and preserve the
existing warning text.
- Allow one-segment provider prefixes with hyphenated provider IDs,
while preserving the multi-segment rejection behavior.
- Add focused coverage for warning dedupe and hyphenated provider-prefix
metadata matching.

Testing:

- Ran `just fmt`.
- Ran `git diff --check`.
- Added tests for the new warning dedupe and provider-prefix lookup
behavior.

codex-rs/models-manager/src/manager_tests.rs

@@ -295,6 +295,21 @@ async fn get_model_info_matches_namespaced_suffix() {
     assert!(!model_info.used_fallback_model_metadata);
 }
 
+#[tokio::test]
+async fn get_model_info_matches_hyphenated_provider_namespace_suffix() {
+    let config = ModelsManagerConfig::default();
+    let remote = remote_model("gpt-image", "Image", /*priority*/ 0);
+    let manager = static_manager_for_tests(ModelsResponse {
+        models: vec![remote],
+    });
+    let namespaced_model = "openai-codex/gpt-image".to_string();
+
+    let model_info = manager.get_model_info(&namespaced_model, &config).await;
+
+    assert_eq!(model_info.slug, namespaced_model);
+    assert!(!model_info.used_fallback_model_metadata);
+}
+
 #[tokio::test]
 async fn get_model_info_rejects_multi_segment_namespace_suffix_matching() {
     let codex_home = tempdir().expect("temp dir");