[13.x] Keep the assertion failure message when session errors are JSON serialized

[lazerg]

Store::save() replaces the in-memory ViewErrorBag with a plain array when the session serializes to JSON, and never puts the object back. TestResponse::session() hides this because it restarts the session, which runs marshalErrorBag() and rebuilds the bag, but TestResponseAssert::injectResponseContext() reads the redirect response's session directly and calls ->all() on whatever is there.

So on a failing assertion against a redirect carrying validation errors, the helper throws Error: Call to a member function all() on array and that replaces the ExpectationFailedException it was meant to enrich. The real failure message, the expected and actual values and the validation messages are all gone, and the trail points at a framework internal rather than the test. It only shows up once an assertion is already failing, so a green suite never surfaces it, and json is the shipped default for new applications.

Starting the session before reading the errors lets the store marshal the bag back, so the assertion message keeps both the original failure and the validation errors.

Fixes #61200