feat: add gemini code sample and test for text-only input (#4130)

## Description

Fixes b:336785030

Please merge on approval

vertexai/snippets/text_input.go

@@ -0,0 +1,53 @@
+// Copyright 2024 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 snippets
+
+// [START generativeaionvertexai_gemini_generate_from_text_input]
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+
+	"cloud.google.com/go/vertexai/genai"
+)
+
+func generateContentFromText(w io.Writer, projectID string) error {
+	location := "us-central1"
+	modelName := "gemini-1.0-pro-vision-001"
+
+	ctx := context.Background()
+	client, err := genai.NewClient(ctx, projectID, location)
+	if err != nil {
+		return fmt.Errorf("error creating client: %w", err)
+	}
+	gemini := client.GenerativeModel(modelName)
+	prompt := genai.Text(
+		"What's a good name for a flower shop that specializes in selling bouquets of dried flowers?")
+
+	resp, err := gemini.GenerateContent(ctx, prompt)
+	if err != nil {
+		return fmt.Errorf("error generating content: %w", err)
+	}
+	// See the JSON response in
+	// https://pkg.go.dev/cloud.google.com/go/vertexai/genai#GenerateContentResponse.
+	rb, err := json.MarshalIndent(resp, "", "  ")
+	if err != nil {
+		return fmt.Errorf("json.MarshalIndent: %w", err)
+	}
+	fmt.Fprintln(w, string(rb))
+	return nil
+}
+
+// [END generativeaionvertexai_gemini_generate_from_text_input]

vertexai/snippets/text_input_test.go

@@ -0,0 +1,34 @@
+// Copyright 2024 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 snippets
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/GoogleCloudPlatform/golang-samples/internal/testutil"
+)
+
+func TestTextInput(t *testing.T) {
+	tc := testutil.SystemTest(t)
+	buf := &bytes.Buffer{}
+	err := generateContentFromText(buf, tc.ProjectID)
+	if err != nil {
+		t.Errorf("generateContentFromText error: %v", err)
+	}
+	if got := buf.String(); !strings.Contains(got, "Candidates") {
+		t.Error("Candidates JSON not found in response")
+	}
+}