Add MockAlertApi and MockFeatureFlagsApi to frontend-test-utils

Introduces comprehensive mock implementations for AlertApi and FeatureFlagsApi in @backstage/frontend-test-utils, following the 3-pattern approach (function, factory, mock) for consistency with existing test utilities.

The new mocks include useful testing methods:
- MockAlertApi: clearAlerts(), waitForAlert(), getAlerts()
- MockFeatureFlagsApi: getState(), setState(), clearState()

Also adds a mockApis namespace that provides these new mocks and re-exports existing mockApis from @backstage/test-utils for backwards compatibility.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

yarn.lock

@@ -3961,9 +3961,11 @@ __metadata:
     "@backstage/version-bridge": "workspace:^"
     "@testing-library/jest-dom": "npm:^6.0.0"
     "@types/react": "npm:^18.0.0"
+    "@types/zen-observable": "npm:^0.8.0"
     react: "npm:^18.0.2"
     react-dom: "npm:^18.0.2"
     react-router-dom: "npm:^6.30.2"
+    zen-observable: "npm:^0.10.0"
     zod: "npm:^3.25.76"
   peerDependencies:
     "@testing-library/react": ^16.0.0