Fix expected server address param name for VPP proxy auth (#38062)

For #32461 to sync server and proxy APIs.

server/mdm/apple/apple_apps/api.go

@@ -275,7 +275,7 @@ func GetAuthenticator(ctx context.Context, ds DataStore, licenseKey string) Auth
 		}
 
 		body, err := json.Marshal(struct {
-			ServerURL string `json:"serverUrl"`
+			ServerURL string `json:"fleetServerUrl"`
 		}{appConfig.ServerSettings.ServerURL})
 		if err != nil {
 			return "", ctxerr.Wrap(ctx, err, "encoding authentication request for VPP metadata service")

server/mdm/apple/apple_apps/api_test.go

@@ -274,7 +274,7 @@ func TestAuthentication(t *testing.T) {
 			// Verify the URL is set
 			body, err := io.ReadAll(r.Body)
 			require.NoError(t, err)
-			require.JSONEq(t, `{"serverUrl": "https://fleet.example.com"}`, string(body))
+			require.JSONEq(t, `{"fleetServerUrl": "https://fleet.example.com"}`, string(body))
 
 			w.WriteHeader(http.StatusOK)
 			_, _ = w.Write([]byte(`{"fleetServerSecret": "new-token-from-auth"}`))
@@ -317,6 +317,7 @@ func TestAuthentication(t *testing.T) {
 		// Set up a mock auth server
 		authServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			require.Equal(t, "Bearer my-license-key", r.Header.Get("Authorization"))
+
 			w.WriteHeader(http.StatusOK)
 			_, _ = w.Write([]byte(`{"fleetServerSecret": "fresh-token"}`))
 		}))