Chat input styling: border radius, icon colors, toolbar tweaks

[daviddossett]

• Use cornerRadius-large for chat input container and attachment bar
• Swap send icon to arrowUp
• Use descriptionForeground for toolbar icons and picker labels
• Add editor view-lines padding
• Update descriptionForeground to #888888 in 2026 dark theme

[Copilot]

Pull request overview

This PR updates the chat input UI styling and labeling to better align with VS Code’s design system tokens (corner radius + foreground colors), and refreshes some toolbar/icon affordances in the chat input experience.

Changes:

• Update chat input container + editing-session container to use --vscode-cornerRadius-large, and adjust chat editor padding.
• Refresh chat execute/queue UI to use Codicon.arrowUp instead of Codicon.send.
• Tweak toolbar/picker colors to use --vscode-descriptionForeground, and add a “Copilot CLI” suffix label for background agent sessions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/vs/workbench/contrib/chat/browser/widget/media/chat.css	Uses corner radius tokens, adjusts chat editor line padding, and overrides toolbar icon/label colors.
src/vs/workbench/contrib/chat/browser/widget/input/sessionTargetPickerActionItem.ts	Adds “Copilot CLI” labeling for background agent session provider in picker UI.
src/vs/workbench/contrib/chat/browser/widget/input/chatQueuePickerActionItem.ts	Replaces send icon with arrow-up icon for primary and “Stop and Send” actions.