Is it normal for a PhD student to be asked to contribute to a grant proposal?

You should have no cause for concern. If the grant is related to your work in any way then it is appropriate for you to be involved in the grant writing.

But you aren't going to "shoot it down". In any reasonable case your work will be reviewed by others putting the grant together and also, especially for a large grant, a grants management office at the university. You might be asked to re-write your contribution once or twice with advice from those others.

It will be a good experience for you. If such large grants are an important part of your field, then you will want to learn what goes in to them so that you can lead it yourself later in your career.

You might try to find writing samples from other grants to aid you. The research office or the PI can probably provide some older work. You can also try to work as closely as possible with others on the team. They likely have more experience than you do and can provide some guidance.

It is completely normal to get people who will be involved with the project to help write the grant. The PI is not going to just take your text and copy-paste it into their proposal without reading it. Doubly so because you've never done this before. The proposal, when it is submitted, will be as good as your group as a whole can make it.

If you want to stay in academia, every time you interview for a faculty position you'll be asked about your experience in applying for grants, because generating income from grants is a key part of what academics do. Having this experience will be very valuable for you.