Test request-based availability checks

Author: erikeldridge

packages/vertexai/src/methods/chrome-adapter.ts

@@ -141,6 +141,25 @@ export class ChromeAdapter {
       }
     }
 
+    if (request.systemInstruction) {
+      const systemContent = request.systemInstruction as Content;
+      // Returns false if the role can't be represented on-device.
+      if (systemContent.role && systemContent.role === 'function') {
+        return false;
+      }
+
+      // Returns false if the system prompt is multi-part.
+      if (systemContent.parts && systemContent.parts.length > 1) {
+        return false;
+      }
+
+      // Returns false if the system prompt isn't text.
+      const systemText = request.systemInstruction as TextPart;
+      if (!systemText.text) {
+        return false;
+      }
+    }
+
     return true;
   }