Give ask-questions carousel a visible box in the Agents window

[romalpani]

Summary

Gives the ask-questions carousel in the Agents window a visible box, so it reads as a bounded surface that belongs to the same "ask the human" family as the tool-call permissions dialog.

Previously the question carousel rendered with only a faint --vscode-chat-requestBorder outline and no fill. In dark themes (and in the Agents window, where the surface is the editor background) it had no visible boundary — only the internal dividers showed.

This is CSS-only — one file, no TypeScript touched.

Before / After

Before---

After---

Changes (chatQuestionCarousel.css)

• Consolidated box styling onto the shared .chat-question-carousel-container:
  • background-color: var(--vscode-panel-background)
  • border: 1px solid var(--vscode-input-border, var(--vscode-chat-requestBorder)) — honors a theme's input border, otherwise falls back to the same visible token the internal dividers use, so the box is always visible.
  • keeps border-radius: var(--vscode-cornerRadius-large) and overflow: hidden.
• Focus treatment: :focus-within / :focus-visible → border-color: var(--vscode-focusBorder).
• Agents window / editor surface override: .agent-sessions-workbench and .editor-instance → background-color: var(--vscode-editor-background).
• De-duplicated the input-part rule down to margin: 0; box styling now lives only on the shared rule, so both render paths (input part + inline fallback) and the Agents-window override apply uniformly.

Consistency with the permissions box

Intentionally aligned with chatToolConfirmationCarousel.css:

Aspect	Questions (this PR)	Permissions (existing)
Border	input-border → chat-requestBorder fallback	1px solid input-border
Radius	cornerRadius-large	cornerRadius-large
Background	panel-background	panel-background
Focus	focus-within → focusBorder	focus-within → focusBorder
Agents/editor surface	editor-background	editor-background
Internal dividers	chat-requestBorder (unchanged)	chat-requestBorder

One deliberate difference: the questions border has a chat-requestBorder fallback because input.border is null/transparent by default — which is exactly why the box wasn't visible before.

Scope / risk

• No behavior change; purely visual.
• No build/type impact (CSS only).

Out of scope

This PR only restyles the box. It does not change carousel behavior — questions still carousel within a single askQuestions call, while separate invocations stack vertically. Aggregating separate invocations into one carousel (as permissions do via ChatToolConfirmationCarouselPart) is a separate, larger change.

[Copilot]

Pull request overview

This PR adjusts the visual styling of the chat “ask-questions” carousel so it reads as a distinct, bounded surface (especially in dark themes and on editor-background surfaces like the Agents window), aligning it with the existing tool-permissions confirmation box styling.

Changes:

• Consolidates the carousel “box” styling onto .chat-question-carousel-container (background fill + border + radius + overflow behavior).
• Adds focus styling via :focus-within / :focus-visible to use --vscode-focusBorder.
• Adds an Agents window / editor-surface override to use --vscode-editor-background for the container background.