workspace: Add trailing / to directories on completion when using OpenPathPrompt (#25430)

Closes #25045

With the setting `"use_system_path_prompts": false`, previously, if the
completion target was a directory, no separator would be added after it,
requiring us to manually append a `/` or `\`. Now, if the completion
target is a directory, a `/` or `\` will be automatically added. On
Windows, both `/` and `\` are considered valid path separators.



https://github.com/user-attachments/assets/0594ce27-9693-4a49-ae0e-3ed29f62526a



Release Notes:

- N/A

crates/file_finder/src/file_finder.rs

@@ -1,5 +1,7 @@
 #[cfg(test)]
 mod file_finder_tests;
+#[cfg(test)]
+mod open_path_prompt_tests;
 
 pub mod file_finder_settings;
 mod new_path_prompt;