Normalize block gap values in layout

[tellthemachines]

What?

Closes #52099.

A little overdue cleanup to ensure block gap values are either strings or numeric zero (if gap is an array, the array should also have only those values). This will hopefully prevent PHP warnings or fatals from incorrect values.

Testing Instructions

1. Check a few different blocks: Columns is a good one because it has axial support, and Group in its various layout types.
2. Set different block gap values on them: units like px, vw... and numeric 0 should all work correctly in the front end.

Use of AI Tools

Used codex/gpt 5.6 sol

[ramonjd]

This is working well for me. No blockers aside from the linting/backport

[github-actions]

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Co-authored-by: tellthemachines <isabel_brison@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

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[andrewserong]

This is working well for me. No blockers aside from the linting/backport

Same here. Good idea adding this hardening!

[github-actions]

Flaky tests detected in d385ee6.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/31562336544
📝 Reported tests:

should update the URL from the last navigation if only varies in the URL fragment in /test/e2e/specs/interactivity/router-navigate.spec.ts, passed after 1 failed attempt.

Error: expect(locator).toHaveText(expected) failed

Locator:  getByTestId('title')
Expected: "Link 1"
Received: "Main"
Timeout:  5000ms

Call log:
  - Expect "toHaveText" with timeout 5000ms
  - waiting for getByTestId('title')
    14 × locator resolved to <h2 data-testid="title">Main</h2>
       - unexpected value "Main"

    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/interactivity/router-navigate.spec.ts:160:25