I guess it's an "keep going" button that you have to hold down, but still, if you're phone freezes - is there a fail state we can be absolutely certain of between the phone and the car?
More than likely, as the driver holds the button on the phone, the phone sends a message with the command, at a decently fast rate. If the message stops coming in, doesn't come in within an acceptable interval, comes in out of order (possibly indicating a bad line of site), or signal strength is low. If any of this happens, the car exits summon and the driver has to re-enable it if they want to.
You hope the car relies on continuous signal from the phone to proceed. It's certainly possible that the phone freezes in a way that the "keep going" signal continues to be sent, but it's probably not the more likely failure mode. Definitely a good thing to be concerned about!
I may be just a bad developer but my first thought when I see or build a new feature for anything is "how could this go wrong, and who will it kill if it does?" Thankfully, I don't make self driving vehicles, so the second question is usually at worst a handful.
Such a thing does not exist. You can only add more nines (e.g. heartbeats, check sensor jitter etc.). At some point you're more likely to be ran over by another car than the tesla slowly, patiently crushing you at 2km/h.
Of course that doesn't mean tesla is actually that reliable.
Heartbeats won't catch UI freezes, which I've seen on phones. Hopefully, those would result in a drop of the input signal rather than it locked in its prior state, though.
Heartbeat would hopefully also include a counter, so that a locked input signal would result in the vehicle rejecting the heartbeat (sees that each message is the same instead of incrementing the counter).
No guarantee that's how they implemented it, though.
I could easily imagine a failure mode where the UI stops responding to input (that is, doesn't pass the "touch end" event to the app) and the app continues to send the "keep going" signal to the car.
That could definitely happen, but it would have to happen at the same time the car's normal collision avoidance systems fail. Certainly possible to happen, but probably pretty unlikely.