[PM-1722] gracefully fail if site prompts user for passkey on load (#6089)

* added error logic to look for options.mediation in page-script

* moved the options mediation logic into the try catch. changed error to FallbackRequestedError

apps/browser/src/vault/fido2/content/page-script.ts

@@ -1,3 +1,5 @@
+import { FallbackRequestedError } from "@bitwarden/common/vault/abstractions/fido2/fido2-client.service.abstraction";
+
 import { WebauthnUtils } from "../../../browser/webauthn-utils";
 
 import { MessageType } from "./messaging/message";
@@ -95,7 +97,12 @@ navigator.credentials.get = async (
   abortController?: AbortController
 ): Promise<Credential> => {
   const fallbackSupported = browserNativeWebauthnSupport;
+
   try {
+    if (options?.mediation && options.mediation !== "optional") {
+      throw new FallbackRequestedError();
+    }
+
     const response = await messenger.request(
       {
         type: MessageType.CredentialGetRequest,