fix: also try matching namespaced prefix for modelinfo candidate (#12658)

#### What
Try matching `\w+`-namespaced model after `longest prefix` as heuristic
to match `ModelInfo` from list of candidates.

This shouldn't regress existing behavior:
- `gpt-5.2-codex` -> `gpt-5.2` if `gpt-5.2-codex` not present
- `gpt-5.3` -> `gpt-5` if `gpt-5.3` not present
- `gpt-9` still doesn't match anything

while being more forgiving for custom prefixes:
- `oai/gpt-5.3-codex` -> `gpt-5.3-codex`

#### Tests
Added unit test.

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

@@ -195,6 +195,65 @@ async fn remote_models_long_model_slug_is_sent_with_high_reasoning() -> Result<(
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn namespaced_model_slug_uses_catalog_metadata_without_fallback_warning() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+    skip_if_sandbox!(Ok(()));
+
+    let server = MockServer::start().await;
+    let requested_model = "custom/gpt-5.2-codex";
+    let response_mock = mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp-1"), ev_completed("resp-1")]),
+    )
+    .await;
+
+    let TestCodex {
+        codex, cwd, config, ..
+    } = test_codex()
+        .with_model(requested_model)
+        .build(&server)
+        .await?;
+
+    codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "check namespaced model metadata".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: cwd.path().to_path_buf(),
+            approval_policy: config.permissions.approval_policy.value(),
+            sandbox_policy: config.permissions.sandbox_policy.get().clone(),
+            model: requested_model.to_string(),
+            effort: None,
+            summary: config.model_reasoning_summary,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+
+    let mut fallback_warning_count = 0;
+    loop {
+        let event = wait_for_event(&codex, |_| true).await;
+        match event {
+            EventMsg::Warning(warning)
+                if warning.message.contains("Defaulting to fallback metadata") =>
+            {
+                fallback_warning_count += 1;
+            }
+            EventMsg::TurnComplete(_) => break,
+            _ => {}
+        }
+    }
+
+    let body = response_mock.single_request().body_json();
+    assert_eq!(body["model"].as_str(), Some(requested_model));
+    assert_eq!(fallback_warning_count, 0);
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn remote_models_remote_model_uses_unified_exec() -> Result<()> {
     skip_if_no_network!(Ok(()));