Fix slash command argument completion bugs (#16233)

Release Notes:

- N/A

---------

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/assistant/src/slash_command/file_command.rs

@@ -164,7 +164,8 @@ impl SlashCommand for FileSlashCommand {
                     Some(ArgumentCompletion {
                         label,
                         new_text: text,
-                        run_command: false,
+                        run_command: true,
+                        replace_previous_arguments: false,
                     })
                 })
                 .collect())