chore(compass-components): push toast behind modal COMPASS-8162

[mabaasit]

Description

Checklist

• New tests and/or benchmarks are included
• Documentation is changed or added
• If this change updates the UI, screenshots/videos are added and a design review is requested
• I have signed the MongoDB Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement)

Motivation and Context

• Bugfix
• New feature
• Dependency update
• Misc

Open Questions

Dependents

Types of changes

• Backport Needed
• Patch (non-breaking change which fixes an issue)
• Minor (non-breaking change which adds functionality)
• Major (fix or feature that would cause existing functionality to change)

[Copilot]

Pull Request Overview

This PR adjusts the z-index behavior of toast notifications to display them behind modal dialogs instead of on top. The change addresses COMPASS-8162 by modifying the toast stacking context calculation.

• Updated z-index calculation to position toasts below modal components
• Renamed variables to better reflect the new behavior
• Updated comments to accurately describe the toast positioning

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[Copilot]

Setting z-index to 0 when no stacked context exists may cause toasts to appear behind other page elements. Consider using a positive default value like 1 to ensure toasts remain visible above normal page content.

Suggested change

	const zIndex = stackedContext?.zIndex ? stackedContext.zIndex - 1 : 0;
	const zIndex = stackedContext?.zIndex ? stackedContext.zIndex - 1 : 1;