supermaven: Fix incorrect offset calculation (#17925)

Fixes a bug in https://github.com/zed-industries/zed/pull/17578 when
computing the offset. Specifically, `offset.add_assign()` should be
incremented on every loop match instead of only when the completion text
is found.

Before:


![image](https://github.com/user-attachments/assets/cc09dbf9-03e8-4453-a1c7-11f838c1d959)

After:


![image](https://github.com/user-attachments/assets/f3513769-d9e1-451f-97dc-b9ad3a57ce3a)

Release Notes:

- Fixed a wrong offset calculation in the Supermaven inline completion
provider.