Global Styles: Respect global border radius opt-out for button block

[BugReportOnWeb]

What?

Closes #56626

This PR ensures that core/button block default settings of border.radius do not override a theme's explicit intent when global support for border is disabled.

Why?

Currently, if a theme sets settings.border.radius to false, the core/button block still shows the border radius UI because core provides a default of true for that block. This leads to confusing behavior where the global opt-out isn't fully respected unless the theme also disables the block level setting manually.

How?

• Introduces a helper function resolve_core_block_default_conflicts() called in get_merged_data()
• This helper checks:
  • If the global border radius is explicitly disabled by the theme.
  • If the block level core/button setting comes only from core (not overridden by theme).
  • If both are true, the core provided core/button.border.radius is unset.

Testing Instructions

1. Use a theme with the following theme.json:

{
	"version": 2,
	"settings": {
		"border": {
			"color": false,
			"radius": false,
			"style": false,
			"width": false
		}
	}
}

2. Open the site editor or post editor and select a button block.
3. Confirm that the Border Radius control does not appear in the block settings panel.

Screenshots or screencast

Using the above provided config for theme.json

Before	After

[t-hamano]

Thanks for the PR! Let's have more discussion before moving forward with this PR. See #56626 (comment)

[github-actions]

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• Labels found: [Block] Buttons, Global Styles.

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[github-actions]

Warning: Type of PR label mismatch

To merge this PR, it requires exactly 1 label indicating the type of PR. Other labels are optional and not being checked here.

• Type-related labels to choose from: [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Technical Prototype, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core, Gutenberg Plugin.
• Labels found: [Block] Buttons, Global Styles.

Read more about Type labels in Gutenberg. Don't worry if you don't have the required permissions to add labels; the PR reviewer should be able to help with the task.