Column: Migrate to width block support

[aaronrobertshaw]

Backport of Gutenberg PR WordPress/gutenberg#76684

Trac ticket: https://core.trac.wordpress.org/ticket/66055

This migrates the Column block to the dimensions.width block support.

Columns are in a flex container and size themselves with flex-basis rather than width, so WP_Theme_JSON converts the generated width declaration and sets flex-grow: 0.

This PR backports:

• flex-basis conversion for column widths in WP_Theme_JSON, covering the block level, block style variations, and variation styles set within a responsive breakpoint
• accompanying WP_Theme_JSON tests.

The core/column block.json and theme.json changes arrive with the next Gutenberg package update, as they did for the Button block in #12046.

Testing instructions

The core/column block.json and theme.json settings changes land with the next Gutenberg package update, so there is no width control in the editor yet.

This PR is the CSS generation, so test that directly.

1. Add a column width to a theme's theme.json:

"styles": {
      "blocks": {
              "core/column": {
                      "dimensions": { "width": "25%" },
                      "@mobile": { "dimensions": { "width": "100%" } }
              }
      }
}

2. Load a page with a Columns block and check the global styles. It should
   contain:

:root :where(.wp-block-column){flex-basis: 25%;flex-grow: 0;}
@media (width <= 480px){:root :where(.wp-block-column){flex-basis: 100%;flex-grow: 0;}}

Without the change these come out as width, which does not size a flex child, so the columns divide the space evenly instead of honouring the width.

Use of AI

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: The mechanical port and test drafting; reviewed and edited by me.

[github-actions]

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