`shinytest2::AppDriver$expect_values()` fails on CI with `invalid argument type` during snapshot

[schloerke]

With an implemented fix for #2288, a invalid argument type is caused at

testthat/R/snapshot-file.R

Line 169 in d695714

if (!equal) {

.

Traceback:

2025-11-21T13:44:32.5296435Z ── Error ('test-app-image.R:46:3'): images are captured via expect_values ──────
2025-11-21T13:44:32.5296659Z Error in `!equal`: invalid argument type
2025-11-21T13:44:32.5296751Z Backtrace:
2025-11-21T13:44:32.5296836Z     ▆
2025-11-21T13:44:32.5297080Z  1. └─app$expect_values() at test-app-image.R:46:3
2025-11-21T13:44:32.5297377Z  2.   └─shinytest2:::app_expect_values(...)
2025-11-21T13:44:32.5297601Z  3.     ├─base::withCallingHandlers(...)
2025-11-21T13:44:32.5297824Z  4.     └─shinytest2:::app_expect_screenshot(...)
2025-11-21T13:44:32.5298068Z  5.       └─shinytest2:::app__expect_snapshot_file(...)
2025-11-21T13:44:32.5298272Z  6.         ├─base::withCallingHandlers(...)
2025-11-21T13:44:32.5298489Z  7.         └─testthat::expect_snapshot_file(...)

I believe this is from {shinytest2}'s want to muffle the failure during snapshot testing so that any screenshot snapshot made during AppDriver$expect_values() will never fail.

I believe the culprit is the method returning the result of snapshot_fail(), which is invisible(). If the failure is muffled, then the return value is NULL.

(PR incoming. Planning to instead continue down the path as if fail_on_new == FALSE)