feat(genai): Add samples for vertexAI express mode api key (GoogleCloudPlatform#5376)

* genai: added vertexAI expressmode api key sample

* genai: PR comments

* genai: PR comments

---------

Co-authored-by: Sampath Kumar <[email protected]>

Author: cfloress

genai/express_mode/expressmode_with_api_key.go

@@ -0,0 +1,65 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package express_mode shows how to use the GenAI SDK to generate text with VertexAI Api Key.
+package express_mode
+
+// [START googlegenaisdk_vertexai_express_mode]
+import (
+	"context"
+	"fmt"
+	"io"
+
+	"google.golang.org/genai"
+)
+
+var newClient = genai.NewClient
+
+// generateContent shows how to use Vertex AI Express mode with an API key.
+func generateContentWithApiKey(w io.Writer) error {
+	ctx := context.Background()
+
+	// TODO(developer): Replace with your actual API key
+	apiKey := "YOUR_API_KEY"
+
+	client, err := newClient(ctx, &genai.ClientConfig{
+		APIKey:      apiKey,
+		HTTPOptions: genai.HTTPOptions{APIVersion: "v1"},
+	})
+	if err != nil {
+		return fmt.Errorf("failed to create genai client: %w", err)
+	}
+
+	modelName := "gemini-2.5-flash"
+	resp, err := client.Models.GenerateContent(ctx, modelName,
+		[]*genai.Content{
+			{Parts: []*genai.Part{
+				{Text: "Explain bubble sort to me."},
+			}, Role: "user"},
+		},
+		nil,
+	)
+	if err != nil {
+		return fmt.Errorf("failed to generate content: %w", err)
+	}
+
+	fmt.Fprintln(w, resp.Text())
+
+	// Example response:
+	// Bubble Sort is a simple sorting algorithm that repeatedly steps through the list
+
+	return nil
+}
+
+// [END googlegenaisdk_vertexai_express_mode]

genai/express_mode/expressmode_with_api_key_test.go

@@ -0,0 +1,57 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package express_mode
+
+import (
+	"bytes"
+	"context"
+	"testing"
+
+	"google.golang.org/genai"
+)
+
+type fakeModels struct{}
+
+func (m *fakeModels) GenerateContent(ctx context.Context, model string, contents []*genai.Content, cfg *genai.GenerateContentConfig) (*genai.GenerateContentResponse, error) {
+	return &genai.GenerateContentResponse{
+		Candidates: []*genai.Candidate{
+			{
+				Content: &genai.Content{
+					Parts: []*genai.Part{{Text: "mocked bubble sort explanation"}},
+				},
+			},
+		},
+	}, nil
+}
+
+func TestExpressModeGenerationWithMockFunctional(t *testing.T) {
+	buf := new(bytes.Buffer)
+	client := &fakeModels{}
+
+	resp, err := client.GenerateContent(context.Background(), "gemini-2.5-flash", nil, nil)
+	if err != nil {
+		t.Fatalf("fake GenerateContent failed: %v", err)
+	}
+
+	buf.WriteString(resp.Candidates[0].Content.Parts[0].Text + "\n")
+
+	got := buf.String()
+	if got == "" {
+		t.Error("expected non-empty mocked output, got empty")
+	}
+	if got != "mocked bubble sort explanation\n" {
+		t.Errorf("unexpected output: got %q", got)
+	}
+}