Huge hugs to you OP.
In answer you your origional question, I would call the police, and if I felt the situation was safe for me to do so I would stop and talk to the person. I have been suicidal, I remember sitting looking at a load of tablets crying because I didn't want to die, I didn't want to leave my family, but I didn't see any other option. Luckily my flatmate was in and he did! 15 years later I'm so glad that I am alive. I would always want to give someone else that chance to recover.
I have also seen many people at the ends of their lives who don't want furthur treatment, who want to slip away peacefully with dignity and I respect that. These are people dying and don't want to prolong their deaths, that is very different from someone who is severely depressed and can recover with treatment, which you can OP.
My DH suffers with serious mental illness, he has made numerous suicide attempts. I remember during his worst crisis he dsapeared and I had no contact with him, he wasn't answering his phone. I became convinced that he had taken his own life and for a short while even wondered if that was for the best considering the hell that he was living through at the time. Thankfully DH is safe and well and with treatment his life is no longer hellish, he is generally a happy person, who very much enjoys being alive.
I know right now life is hell, but this will pass. This is the rock bottom, the only way from here is up. There are a lot of people who care for you OP.