feat(llm): support OpenAI Responses API via per-provider api_type

pkg/config/config_test.go

@@ -370,3 +370,74 @@ providers:
 		t.Fatalf("err = %v", err)
 	}
 }
+
+func TestLoadDefaultsAPITypeToChat(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("AGENTU_TEST_KEY", "sk-test")
+	path := filepath.Join(dir, "agentu.yaml")
+	if err := os.WriteFile(path, []byte(`
+providers:
+  test:
+    base_url: https://example.com
+    api_key: ${AGENTU_TEST_KEY}
+    models:
+      - id: test-model
+`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := cfg.Providers["test"].APIType; got != APITypeChat {
+		t.Fatalf("api type = %q, want %q", got, APITypeChat)
+	}
+}
+
+func TestLoadParsesResponsesAPIType(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("AGENTU_TEST_KEY", "sk-test")
+	path := filepath.Join(dir, "agentu.yaml")
+	if err := os.WriteFile(path, []byte(`
+providers:
+  openai:
+    api_type: RESPONSES
+    base_url: https://api.openai.com
+    api_key: ${AGENTU_TEST_KEY}
+    models:
+      - id: gpt-test
+`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := cfg.Providers["openai"].APIType; got != APITypeResponses {
+		t.Fatalf("api type = %q, want %q", got, APITypeResponses)
+	}
+}
+
+func TestLoadRejectsInvalidAPIType(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("AGENTU_TEST_KEY", "sk-test")
+	path := filepath.Join(dir, "agentu.yaml")
+	if err := os.WriteFile(path, []byte(`
+providers:
+  test:
+    api_type: completions
+    base_url: https://example.com
+    api_key: ${AGENTU_TEST_KEY}
+    models:
+      - id: test-model
+`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := Load(path)
+	if err == nil || !strings.Contains(err.Error(), "api_type") {
+		t.Fatalf("err = %v", err)
+	}
+}