Your ideas are valueless. (Sorry!) My ideas are valueless too. There are approximately 197 billion story ideas floating around the litosphere just waiting for someone with a net to scoop them up.
What is valuable is your ability to take a literary idea and turn it into an interesting story. People who can do that are rare and difficult to find. I don't know if you are one of those people or not. (I don't even know if I am either.) But I do know this:
Agents are not looking for stories, or for ideas, agents are looking for writers. Agents and publishers don't make money (or not reliably) off individual stories. They make money off writer's careers.
This is why an agent is just as interested in your ability to deliver professional work consistently and on time as they are in your ability to write an interesting story. They care about your dedication and reliability as well as your talent.
All of which adds up to, Agents are looking for commercially viable writers, and commercially viable writers are hard to find. If they think you are a commercially viable writer, they will want to represent you, not your story, but you. If they don't think you are a commercially viable writer, they will have no interest in your ideas either.
And to be frank, even if someone does come along and hear your idea and goes and makes a best selling novel out of it, you have lost nothing, because if you are a commercially viable writer, you can generate as many story ideas as you need, and if you are not, you were never going to realize any money off that idea anyway.
Ideas are dross. It is your vision and your talent that will make you a successful writer, and no one can steal either of those.