Move customization list scrollbar to pane edge

[Paul (pwang347)]

Summary

• move the customizations list, migration page, and Tools section scrollbars to the outer pane edge
• make each panel own vertical spacing while its header, controls, and content own their horizontal inset
• give virtualized pages dedicated native scroll containers around their content roots
• preserve the existing centered maximum width without negative margins or synthetic width expansion
• fix the clipped Extension Tools section so every tool set remains reachable
• strengthen narrow fixtures with independently scrollable outer and nested lists

Fixes #335508

Verification

• npm run stylelint -- src/vs/workbench/contrib/chat/browser/aiCustomization/media/aiCustomizationManagement.css
• npm run eslint -- src/vs/workbench/contrib/chat/browser/aiCustomization/{aiCustomizationManagementEditor,toolsListWidget}.ts
• npm run eslint -- src/vs/workbench/test/browser/componentFixtures/sessions/aiCustomizationManagementEditor.fixture.ts
• 69 focused customization editor/list/Tools unit tests
• all migration fixtures, plus Agents standard/narrow, Skills, Tools standard/narrow/light/empty/browse, and MCP regression fixtures
• customization and migration nested-scroll assertions: Workspace items, outer sections, and User items scroll independently
• Extension Tools assertion: outer sections reach Extension Tools, its nested list reaches exact end, and Tool Set 8 is visible

After

Customization list — outer and nested scrollbars

customizations-scrollbar-after-final.mp4

Migrations dashboard and workspace review scrollbars

merged-migration-scrollbars-final.mp4

Extension Tools — #335508

extension-tools-overflow-final.mp4

[Copilot]

Copilot review overview

🟢 Approval recommended

The scoped CSS correctly preserves content alignment while relocating the scrollbar without introducing accessibility or layout issues.

Review tier: Balanced
Findings: None

What changed in this PR

Moves the customization list scrollbar to the pane edge while preserving centered, inset content.

Changes:

• Expands the list widget to the pane edges.
• Retains the existing 840px content limit and inset alignment.

File	Description
aiCustomizationManagement.css	Adjusts list and scroll-container sizing.

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