[generative_ai] Missed regressions in the code samples because of targeted test runs

## Describe the issue

We're starting to miss regressions in the code samples for `generative_ai` because of migrating the CI to do targeted (per feature directory) test runs. Meaning, when there are no modifications done in a feature directory, the tests will not run in this directory and the issue will silently live there, potentially for a long time.

We need to run the entire test suite regularly to ensure all code samples remain functional.

## Examples

1. [function_calling/basic_example.py](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/generative_ai/function_calling/basic_example.py#L42) is not working anymore with Gemini Flash model. I discovered this when updated a different sample in this directory (see [the comment](https://github.com/GoogleCloudPlatform/python-docs-samples/pull/12811/files#r1866272343)).
2. Two function calling with OpenAI samples — [function_calling/chat_function_calling_basic.py](https://github.com/GoogleCloudPlatform/python-docs-samples/tree/main/generative_ai/function_calling/chat_function_calling_basic.py) & [function_calling/chat_function_calling_config.py](https://github.com/GoogleCloudPlatform/python-docs-samples/tree/main/generative_ai/function_calling/chat_function_calling_config.py) — are not working anymore. Same story here: I discovered the issue only because I updated a different file in this feature directory. See [this comment](https://github.com/GoogleCloudPlatform/python-docs-samples/pull/12811/files#r1866447860).