Fix slash command argument completion bugs (#16233)

Release Notes:

- N/A

---------

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

crates/assistant/src/slash_command/diagnostics_command.rs

@@ -150,6 +150,7 @@ impl SlashCommand for DiagnosticsSlashCommand {
                     label: completion.clone().into(),
                     new_text: completion,
                     run_command: true,
+                    replace_previous_arguments: false,
                 })
                 .collect())
         })