Enhance accessibility for ModelSelector dropdown

Added keyboard support for toggling the dropdown using Enter or Space keys, improving accessibility. Also set appropriate focus properties by adding tabindex to the combobox element.

app/components/chat/ModelSelector.tsx

@@ -186,10 +186,17 @@ export const ModelSelector = ({
             isModelDropdownOpen ? 'ring-2 ring-bolt-elements-focus' : undefined,
           )}
           onClick={() => setIsModelDropdownOpen(!isModelDropdownOpen)}
+          onKeyDown={(e) => {
+            if (e.key === 'Enter' || e.key === ' ') {
+              e.preventDefault();
+              setIsModelDropdownOpen(!isModelDropdownOpen);
+            }
+          }}
           role="combobox"
           aria-expanded={isModelDropdownOpen}
           aria-controls="model-listbox"
           aria-haspopup="listbox"
+          tabIndex={0}
         >
           <div className="flex items-center justify-between">
             <div className="truncate">{modelList.find((m) => m.name === model)?.label || 'Select model'}</div>