Fix more bugs in files (#16241)

Fixes:
- [x] an issue where directories would only match by prefix, causing
both a directory and a file to be matched if in the same directory
- [x] An issue where you could not continue a file completion when
selecting a directory, as `tab` on a file would always run the command.
This effectively disabled directory sub queries.
- [x] Inconsistent rendering of files and directories in the slash
command

Release Notes:

- N/A

---------

Co-authored-by: max <max@zed.dev>

crates/assistant/src/slash_command/prompt_command.rs

@@ -45,7 +45,7 @@ impl SlashCommand for PromptSlashCommand {
                     Some(ArgumentCompletion {
                         label: prompt_title.clone().into(),
                         new_text: prompt_title,
-                        run_command: true,
+                        after_completion: true.into(),
                         replace_previous_arguments: true,
                     })
                 })