fix(a11y): add aria-labels to chat message components (#21708)

Add aria-labels, aria-expanded, and semantic improvements to:
- RateComment: close button, rating scale, feedback textarea
- Citations: toggle button with count, source item buttons
- Source/SourceToken: contextual aria-labels for citation buttons
- StatusHistory: toggle button with expanded state
- WebSearchResults: descriptive favicon alt text
- FollowUps: convert div to button element
- RegenerateMenu: submit suggestion button
- FloatingButtons: action buttons, input field, submit button
- CitationModal: close button

WCAG: 4.1.2 (Name, Role, Value), 2.1.1 (Keyboard), 1.1.1 (Non-text Content)

src/lib/components/chat/Messages/ResponseMessage/RegenerateMenu.svelte

@@ -53,13 +53,14 @@
 
 				<div class="ml-2 self-center flex items-center">
 					<button
-						class="{inputValue !== ''
-							? 'bg-black text-white hover:bg-gray-900 dark:bg-white dark:text-black dark:hover:bg-gray-100 '
-							: 'text-white bg-gray-200 dark:text-gray-900 dark:bg-gray-700 disabled'} transition rounded-full p-1 self-center"
-						on:click={() => {
-							onRegenerate(inputValue);
-							show = false;
-						}}
+					aria-label={$i18n.t('Submit suggestion')}
+					class="{inputValue !== ''
+						? 'bg-black text-white hover:bg-gray-900 dark:bg-white dark:text-black dark:hover:bg-gray-100 '
+						: 'text-white bg-gray-200 dark:text-gray-900 dark:bg-gray-700 disabled'} transition rounded-full p-1 self-center"
+					on:click={() => {
+						onRegenerate(inputValue);
+						show = false;
+					}}
 					>
 						<svg
 							xmlns="http://www.w3.org/2000/svg"