docs(generative_ai): Add Vertex AI Express Mode Sample (#12988)

* docs(generative_ai): Add Vertex AI Express Mode Sample

- https://cloud.google.com/vertex-ai/generative-ai/docs/start/express-mode/overview

---------

Co-authored-by: Sampath Kumar <sampathm@google.com>
Co-authored-by: Maciej Strzelczyk <strzelczyk@google.com>

generative_ai/express_mode/api_key_example.py

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+# 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.
+
+
+def generate_content() -> None:
+    # [START generativeaionvertexai_gemini_express_mode]
+    import vertexai
+    from vertexai.generative_models import GenerativeModel
+
+    # TODO(developer): Update below line
+    vertexai.init(api_key="YOUR_API_KEY")
+
+    model = GenerativeModel("gemini-1.5-flash")
+
+    response = model.generate_content("Explain bubble sort to me")
+
+    print(response.text)
+    # Example response:
+    # Bubble Sort is a simple sorting algorithm that repeatedly steps through the list
+    # [END generativeaionvertexai_gemini_express_mode]
+    return response.text

generative_ai/express_mode/api_key_example_test.py

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+# 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.
+
+from unittest.mock import MagicMock, patch
+
+from vertexai.generative_models import (
+    GenerationResponse,
+    GenerativeModel,
+)
+
+import api_key_example
+
+
+@patch.object(GenerativeModel, "generate_content")
+def test_api_key_example(mock_generate_content: MagicMock) -> None:
+    # Mock the API response
+    mock_generate_content.return_value = GenerationResponse.from_dict(
+        {
+            "candidates": [
+                {
+                    "content": {
+                        "parts": [{"text": "This is a mocked bubble sort explanation."}]
+                    }
+                }
+            ]
+        }
+    )
+
+    # Call the function
+    response = api_key_example.generate_content()
+
+    # Assert that the function returns the expected value
+    assert response == "This is a mocked bubble sort explanation."

generative_ai/express_mode/noxfile_config.py

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+# Copyright 2021 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.
+
+# Default TEST_CONFIG_OVERRIDE for python repos.
+
+# You can copy this file into your directory, then it will be imported from
+# the noxfile.py.
+
+# The source of truth:
+# https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/noxfile_config.py
+
+TEST_CONFIG_OVERRIDE = {
+    # You can opt out from the test for specific Python versions.
+    "ignored_versions": ["2.7", "3.7", "3.8", "3.10", "3.11", "3.13"],
+    # Old samples are opted out of enforcing Python type hints
+    # All new samples should feature them
+    "enforce_type_hints": True,
+    # An envvar key for determining the project id to use. Change it
+    # to 'BUILD_SPECIFIC_GCLOUD_PROJECT' if you want to opt in using a
+    # build specific Cloud project. You can also use your own string
+    # to use your own Cloud project.
+    "gcloud_project_env": "GOOGLE_CLOUD_PROJECT",
+    # 'gcloud_project_env': 'BUILD_SPECIFIC_GCLOUD_PROJECT',
+    # If you need to use a specific version of pip,
+    # change pip_version_override to the string representation
+    # of the version number, for example, "20.2.4"
+    "pip_version_override": None,
+    # A dictionary you want to inject into your test. Don't put any
+    # secrets here. These values will override predefined values.
+    "envs": {},
+}

generative_ai/express_mode/requirements-test.txt

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+google-api-core==2.24.0
+pytest==8.2.0

generative_ai/express_mode/requirements.txt

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+google-cloud-aiplatform==1.74.0