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/ContentRenderer/FloatingButtons.svelte

@@ -254,6 +254,7 @@
 			>
 				{#each actions as action}
 					<button
+						aria-label={action.label}
 						class="px-1.5 py-[1px] hover:bg-gray-50 dark:hover:bg-gray-800 rounded-xl flex items-center gap-1 min-w-fit transition"
 						on:click={async () => {
 							selectedText = window.getSelection().toString();
@@ -291,6 +292,7 @@
 					id="floating-message-input"
 					class="ml-5 bg-transparent outline-hidden w-full flex-1 text-sm"
 					placeholder={$i18n.t('Ask a question')}
+					aria-label={$i18n.t('Ask a question')}
 					bind:value={floatingInputValue}
 					on:keydown={(e) => {
 						if (e.key === 'Enter') {
@@ -301,6 +303,7 @@
 
 				<div class="ml-1 mr-1">
 					<button
+						aria-label={$i18n.t('Submit question')}
 						class="{floatingInputValue !== ''
 							? '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-1.5 m-0.5 self-center"