assistant: Fix evaluating slash commands in slash command output (like /default) (#20864)

This PR fixes an issue where slash commands in the output of other slash
commands were not being evaluated when configured to do so.

Closes https://github.com/zed-industries/zed/issues/20820.

Release Notes:

- Fixed slash commands from other slash commands (like `/default`) not
being evaluated (Preview only).

crates/assistant/src/slash_command/default_command.rs

@@ -69,6 +69,10 @@ impl SlashCommand for DefaultSlashCommand {
                 text.push('\n');
             }
 
+            if !text.ends_with('\n') {
+                text.push('\n');
+            }
+
             Ok(SlashCommandOutput {
                 sections: vec![SlashCommandOutputSection {
                     range: 0..text.len(),