e2e: add models test using the openai client (#338)

**Commit Message**

e2e: add test for the models endpoint using the openai client

**Related Issues/PRs (if applicable)**

Related to #325

**Special notes for reviewers (if applicable)**

N/A

---------

Signed-off-by: Ignasi Barrera <ignasi@tetrate.io>

tests/extproc/real_providers_test.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/openai/openai-go"
 	"github.com/openai/openai-go/option"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"github.com/envoyproxy/ai-gateway/filterapi"
@@ -230,6 +231,29 @@ func TestWithRealProviders(t *testing.T) {
 			return returnsToolCall
 		}, 30*time.Second, 2*time.Second)
 	})
+
+	// Models are served by the extproc filter as a direct response so this can run even if the
+	// real credentials are not present.
+	// We don't need to run it on a concrete backend, as it will not route anywhere.
+	t.Run("list-models", func(t *testing.T) {
+		client := openai.NewClient(option.WithBaseURL(listenerAddress + "/v1/"))
+
+		var models []string
+
+		require.EventuallyWithT(t, func(c *assert.CollectT) {
+			it := client.Models.ListAutoPaging(t.Context())
+			for it.Next() {
+				models = append(models, it.Current().ID)
+			}
+			assert.NoError(c, it.Err())
+		}, 30*time.Second, 2*time.Second)
+
+		require.Equal(t, []string{
+			"gpt-4o-mini",
+			"us.meta.llama3-2-1b-instruct-v1:0",
+			"us.anthropic.claude-3-5-sonnet-20240620-v1:0",
+		}, models)
+	})
 }
 
 // realProvidersTestCase is a base test case for the real providers, which is mainly for the centralization of the