fix(py): fix broken flows in short-n-long sample

[MengqinShen]

Update the Genkit framework's serialization utility (dump_dict) in the core framework (py/packages/genkit/src/genkit/codec.py) to handle lists and dicts recursively. This is a better fix than modifying the sample, as it benefits all flows returning lists of models.

[gemini-code-assist]

Summary of Changes

Hello @MengqinShen, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses issues related to data serialization and model usage within the genkit Python package. It enhances the dump_dict function to recursively handle Pydantic models within complex data structures, ensuring robust data conversion. Additionally, it updates a sample application to utilize newer Gemini models and introduces dynamic prompting for image generation, improving the sample's functionality and demonstrating current best practices.

Highlights

• Recursive Serialization Fix: The dump_dict utility now correctly serializes Pydantic BaseModel instances embedded within lists and dictionaries, ensuring proper data conversion for complex structures.
• Enhanced Test Coverage: New unit tests have been added for dump_dict to validate its recursive handling of lists of models and nested model structures, preventing regressions.
• Model Update in Sample: The short-n-long sample application has been updated to use the gemini-3-pro-preview model by default, aligning with newer model versions.
• Dynamic Image Generation Prompt: The generate_images function in the sample now uses gemini-3-pro-image-preview and constructs a dynamic prompt based on user input, improving flexibility.

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[gemini-code-assist]

Code Review

This pull request fixes an issue in dump_dict where it didn't correctly serialize Pydantic models nested within lists or dictionaries. The change adds recursive handling for these container types. Corresponding unit tests have been added to verify the fix. Additionally, a sample application has been updated to use a newer model and to make a function more generic. My main feedback is to add cycle detection to the new recursive logic in dump_dict to prevent potential stack overflows with self-referencing data structures.