Add visible focus indicator to MCP elicitation multi-select options

[Copilot]

Description

In the MCP elicitation form (rendered by the chat question carousel), keyboard navigation through multi-select options moved focus with no visible focus indicator, so keyboard users couldn't tell which option was focused (WCAG 2.4.3).

The two selection types manage focus differently:

• Single-select keeps DOM focus on the list container and uses aria-activedescendant + the .selected class — already visible.
• Multi-select moves real DOM focus onto each role="option" item via listItems[focusedIndex].focus(). The CSS rule .chat-question-list-item:focus { outline: none } stripped the indicator, leaving focused-but-unchecked items with no visual cue.

Changes

• chatQuestionCarousel.css: split the combined outline: none rule. The list container still suppresses its outline (relies on aria-activedescendant), while focused list items now render a --vscode-focusBorder outline:

.chat-question-list-item:focus {
	outline: 1px solid var(--vscode-focusBorder);
	outline-offset: -1px;
}

Uses the standard focus-border token (consistent with the freeform textarea styling in the same file), so high-contrast and custom themes are respected. Single-select is unaffected since its items never take direct DOM focus.