feat: update region tags for existing Imagen samples

[irataxy]

Description

Fixes b:329291575

• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• generative_ai/imagen/edit_image_mask.py:24, tag generativeaionvertexai_imagen_edit_image_mask
• generative_ai/imagen/edit_image_mask_free.py:23, tag generativeaionvertexai_imagen_edit_image_mask_free
• generative_ai/imagen/get_short_form_image_captions.py:22, tag generativeaionvertexai_imagen_get_short_form_image_captions
• generative_ai/imagen/get_short_form_image_responses.py:23, tag generativeaionvertexai_imagen_get_short_form_image_responses

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[m-strzelczyk]

To get rid of the snippet bot complaints, just make it double-region tagged. It's OK to have two region tags overlap. Then, when the docs are updated to use the new tag, you'll be able to remove the old tags from here.

[irataxy]

To get rid of the snippet bot complaints, just make it double-region tagged. It's OK to have two region tags overlap. Then, when the docs are updated to use the new tag, you'll be able to remove the old tags from here.

Done