gpui: Add support for custom prompt text in PathPromptOptions (#36410)

This will be used to improve the clarity of the git clone UI

### MacOS
<img width="1322" height="128" alt="image"
src="https://github.com/user-attachments/assets/3e511143-12c1-4440-89dd-841b21b2e98e"
/>

### Windows 
<img width="338" height="80" alt="image"
src="https://github.com/user-attachments/assets/766d08d6-0c72-4175-ad24-59dc6188d5f1"
/>

### Linux

<img width="387" height="72" alt="Screenshot From 2025-08-18 15-32-06"
src="https://github.com/user-attachments/assets/3125a7c4-3975-462a-a547-d5d4fac48f22"
/>



Release Notes:

- N/A

crates/gpui/src/platform/linux/platform.rs

@@ -294,6 +294,7 @@ impl<P: LinuxClient + 'static> Platform for P {
                 let request = match ashpd::desktop::file_chooser::OpenFileRequest::default()
                     .modal(true)
                     .title(title)
+                    .accept_label(options.prompt.as_ref().map(crate::SharedString::as_str))
                     .multiple(options.multiple)
                     .directory(options.directories)
                     .send()