feat: Nano Banana GA and video input samples

[katiemn]

Description

Add samples for video-to-image with Nano Banana 2 and update Nano Banana models

[snippet-bot]

Here is the summary of changes.

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• genai/image_generation/imggen_mmflash_img_with_vid.py:17, tag googlegenaisdk_imggen_mmflash_img_with_vid

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[gemini-code-assist]

Code Review

This pull request updates several image generation examples to use the "gemini-3.1-flash-image" model and introduces a new example demonstrating image generation using a video source. A critical issue was identified in the new video-based image generation script where the Image class is incorrectly imported from google.genai.types instead of PIL, which will cause a runtime AttributeError when attempting to call Image.open().

[iennae]

note: validated that this was the correct model

[iennae]

validating based on policy guidance from Ankur and review of the code