Closes #33152.

The label for a file selection context had an off-by-one error on line
indices. This PR adjusts that.

Before:

<img width="1072" alt="Screenshot 2025-06-27 at 20 55 28"
src="https://github.com/user-attachments/assets/da0be503-056b-442b-9a79-38cf504d8a44"
/>

After:

<img width="1090" alt="Screenshot 2025-06-27 at 20 49 35"
src="https://github.com/user-attachments/assets/f98df57a-8f07-4ea5-a06b-a4b6fb8c2e4e"
/>


Release Notes:

- Fixed a off-by-one error on the line indices when using a selection as
context for the agent,