It depends on the therapist. Some are willing and able to discuss suicide, and won't do anything unless you say you have a plan and intend to act on it in the near future. But this is the exception. Many therapists freak out when someone expresses suicidal ideation, and want to cover their asses in the unlikely chance the client CTBs. From the therapist's perspective, he has a clean conscious and avoids all legal liability by committing you. He might lose a client, but there is a line out the door for replacements, because people have been sold the lie that therapy works.
In many places (including the United States), therapists are unable to involuntarily commit you without another so-called "professional" examining you, but this other person nearly always rubber stamps commitment if it is a 72-hour hold. So don't think you'll be protected from the whims of your therapist.
I don't think they are the exception. Sure, as you are saying, if you talk about how you are going to do it, or about really specific methods, like SN, that you wouldn't know unless you researched a lot, they may do it.
But you have to think that, even if talking about suicide is a taboo, suicide ideation is not uncommon. You say that they would freak out, but therapists deal with suicidal ideation every day. You say that they may lose a client, but they have a line out the door, but most of them have suicidal ideation too. It simply makes no sense that they would send all of them to a ward.
In my opinion, if you don't talk too much about suicide (talking about plans, about SaSu, about SN or whatever...) and you don't reject treatment/meds, you should be safe, especially if they see you as a mature, cooperative and kinda stable adult. After all, if you were going to CTB tomorrow, you wouldn't be looking for a new therapist. Tell them that you thought about death, but you never thought about really doing it, if you want. Or just completely lie, if you want, and tell them that you didn't think about it. You can be honest after some months, if you prefer and if you feel comfortable with them (and if you don't feel comfortable with them, just leave and look for someone else).
Edit: even if I said that they are not the exception, that doesn't mean that there aren't a lot of therapists like that. A lot of them are incompetent or even abusive, and this may be your case, Johnzaga23.