assistant: Fix nested slash command rendering (#16173)

/rant on
We have this issue where if a prompt starts with a slash command (e.g.
/workflow), the rendering is a bit messed up. The nested slash command
gets picked up as the parent of a command that includes it (/prompt).
This is due to how we parse slash commands; their output is obtained
asynchronously. When we run `/prompt "My prompt"` whose contents are
`/workflow`, we first include the prompt content verbatim and then
reparse the whole buffer, picking up /workflow as a new command (as if
it was typed by an user). The problem with that is that the range of
parent /prompt does not include the expanded range of a /workflow; in
fact, after doing full expansion of "My prompt", we lose track of the
parent-children relationship of these two slash commands and treat them
as if top-level user prompt was `/workflow/prompt "My prompt"` and not
`/prompt "My prompt"` (which, by the way, would not be parsable for us).

The "proper" fix would be to update the parent range whenever we parse a
new children within it. We could do that. But then, the question is;
what do we gain from it? Slash command output is put behind a crease,
which is fundamentally a fold. Given "My prompt", we'd have to put two
fold indicators on a single line even if the ranges were set up
correctly. So that merely moves the target elsewhere into yet another
issue. Even if we did solve two-fold problem somehow (by e.g. sorting
same-line folds by the end point), we would still be stuck with
suboptimal render. What do we gain from all that anyways? Proper
handling of a relatively obscure (although - at the same time - quite
common) edge case which may as well be handled by having /prompt insert
a new line if there's a slight chance that the edge case could occur.
And that hacky, "inproper" solution is what this PR does; in fact, it's
not the first time it was done, as /default also has the same issue
which it solves in precisely the same manner.

/rant off

Release Notes:

- N/A

crates/assistant/src/slash_command/prompt_command.rs

@@ -77,6 +77,11 @@ impl SlashCommand for PromptSlashCommand {
         });
         cx.foreground_executor().spawn(async move {
             let mut prompt = prompt.await?;
+
+            if prompt.starts_with('/') {
+                // Prevent an edge case where the inserted prompt starts with a slash command (that leads to funky rendering).
+                prompt.insert(0, '\n');
+            }
             if prompt.is_empty() {
                 prompt.push('\n');
             }