`text` definition have logic problem in the `class GenerationResponse`

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#### Environment details

- OS type and version: Debian GNU/Linux forky/sid
- Python version: 3.12.10
- pip version: 25.0.1
- `google-cloud-aiplatform` version: 1.132.0

#### Steps to reproduce

1. run `https://github.com/GoogleCloudPlatform/applied-ai-engineering-samples/blob/main/genai-on-vertex-ai/gemini/prompting_recipes/pdf_processing/1_extract_pdf_pages_with_gemini.ipynb`

#### Code example

In the `1_extract_pdf_pages_with_gemini.ipynb`:

```python
with open(pdf_path, "rb") as f:
pdf_bytes = f.read()
pages = pdf_pages(question=question, pdf_bytes=pdf_bytes)
print(pages)
```

#### Stack trace
```
Traceback (most recent call last):
File "/mnt/e/workspace/github.com/anlexN/applied-ai-engineering-samples/genai-on-vertex-ai/gemini/prompting_recipes/pdf_processing/untitled.py", line 162, in
pages = pdf_pages(question=question, pdf_bytes=pdf_bytes)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/mnt/e/workspace/github.com/anlexN/applied-ai-engineering-samples/genai-on-vertex-ai/gemini/prompting_recipes/pdf_processing/untitled.py", line 147, in pdf_pages
output_json = json.loads(responses.text)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/anlexn/.pyenv/versions/3.12.10/lib/python3.12/json/__init__.py", line 346, in loads
return _default_decoder.decode(s)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/anlexn/.pyenv/versions/3.12.10/lib/python3.12/json/decoder.py", line 338, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/anlexn/.pyenv/versions/3.12.10/lib/python3.12/json/decoder.py", line 356, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

I have opened [**pull request**](https://github.com/googleapis/python-aiplatform/pull/6217), please review.

Bắt đầu với genai-on-vertex-ai/gemini/prompting_recipes/pdf_processing/1_extract_pdf_pages_with_gemini.ipynb và tái hiện JSONDecodeError tại json.loads(responses.text). Sau đó, kiểm tra định nghĩa của GenerationResponse và pull request #6217 được liên kết; công việc được xem là hoàn tất khi notebook tạo ra một response có thể phân tích mà không gặp lỗi này.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.