Spacer: Respect disabled custom spacing sizes when resizing

[salvatorecorsi]

What?

Part of #54084.

Hide Spacer resize controls when the editor's disableCustomSpacingSizes setting is enabled, for both vertical and horizontal Spacers.

Why?

A theme can set settings.spacing.customSpacingSize to false to limit spacing to presets. The Spacer's resize handles currently remain available and convert preset dimensions into custom pixel values when dragged.

How?

Use the setting already read by SpacerEdit when deciding whether to render its ResizableSpacer. Preset controls remain available, and resizing remains available when custom spacing sizes are allowed.

Add E2E coverage for vertical Spacers and Spacers inside a Row, checking preset preservation after saving and reloading, preset selection for the vertical Spacer, and dragging after custom spacing sizes are enabled again.

This addresses the resize interactions described in #54084. The separate request to change the initial 100px default is outside this PR.

Testing Instructions

1. Use a theme with spacing presets, such as Twenty Twenty-Five, and set settings.spacing.customSpacingSize to false in its theme.json.
2. Insert a Spacer in a post, choose a preset for Height, and select the block. Verify that the bottom resize handle is absent.
3. Choose another Height preset, save the draft, and reload. Verify that the selected preset is preserved and the resize handle remains absent.
4. Insert a Row containing a Spacer. Select the Spacer and verify that its horizontal resize handle is absent.
5. Change settings.spacing.customSpacingSize to true and reload. Verify that both Spacers can be resized by dragging their respective handles.

Testing Instructions for Keyboard

With custom spacing sizes disabled, select the vertical Spacer using List View, then navigate to its Height preset slider in the block settings. Use the arrow keys to select another preset, save the draft, and reload. Verify that the chosen preset remains selected.

Validation

• Production build.
• JavaScript type checking and scoped lint/format checks.
• Manual verification in the local post editor.
• Both new Chromium Spacer E2E regression cases passed on GitHub Actions on commit 3bf249c2, on their first attempt (retry 0): vertical 3.651s, horizontal 3.711s.
• The eight other CI workflows passed, including static analysis, unit tests, production builds, performance tests, and Storybook.

E2E suite status

Both new Spacer cases passed on their first attempt. The general E2E suite currently has failures in two unchanged tests, running in separate WordPress/browser instances from the Spacer tests:

• Classic, shard 4: should insert media, convert to blocks, and undo in one step, classic.spec.js:69. The checkbox locator matches the selected image in both the Media Library and attachment selection, causing a strict-mode violation.
• Widgets Customizer, shard 7: should handle legacy widgets, customizing-widgets.spec.js:385. The “Test Widget” heading is absent after the assertion timeout, including both retries.

Both assertions are unchanged from the base commit.

Use of AI Tools

OpenAI Codex was used to investigate the issue, implement the change, write the regression tests, and run validation.

[github-actions]

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[lancewillett]

Classic, shard 4: should insert media, convert to blocks, and undo in one step, classic.spec.js:69. The checkbox locator matches the selected image in both the Media Library and attachment selection, causing a strict-mode violation.

Opened #84520 with the minimal selector fix.