Gallery: Allow gallery to have more columns than total images

[dhananjaykuber]

Fixes #55231

What?

This PR modifies the Gallery block to allow setting the number of columns independently from the number of images, removing the current restriction where column count cannot exceed image count.

Why?

Currently, the Gallery block does not allow setting more columns than there are images. This creates inconsistencies in layouts where multiple galleries need to maintain the same column width, particularly when displaying content like candidate photos where equal sizing is important for fairness.

How?

• Modified the column width calculation in gallery styles to properly account for gaps
• Removed the UI restriction that prevented setting columns > number of images
• Fixed flex-grow behavior to prevent images from expanding to fill empty columns
• Updated width calculations to maintain consistent image sizes across different gallery configurations

Testing Instructions

1. Create a new post
2. Insert a Gallery block
3. Add 2 images to the gallery
4. Try setting the number of columns to 4
5. Verify that the 2 images maintain consistent sizes and don't expand to fill empty spaces
6. Add another gallery below with 4 images and set to 4 columns
7. Verify that images in both galleries are the same size despite different image counts

Screenshots or screencast

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Co-authored-by: dhananjaykuber <dhananjaykuber@git.wordpress.org>
Co-authored-by: cbirdsong <cbirdsong@git.wordpress.org>

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