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/diagnostics_command.rs

@@ -153,7 +153,7 @@ impl SlashCommand for DiagnosticsSlashCommand {
                 .map(|completion| ArgumentCompletion {
                     label: completion.clone().into(),
                     new_text: completion,
-                    run_command: true,
+                    after_completion: assistant_slash_command::AfterCompletion::Run,
                     replace_previous_arguments: false,
                 })
                 .collect())