Rework prompt frontmatter (#12262)

Moved some things around so prompts now always have front-matter to
return, either by creating a prompt with default front-matter, or
bailing earlier on importing the prompt to the library.

In the future we'll improve visibility of malformed prompts in the
`prompts` folder in the prompt manager UI.

Fixes:

- Prompts inserted with the `/prompt` command now only include their
body, not the entire file including metadata.
- Prompts with an invalid title will now show "Untitled prompt" instead
of an empty line.

Release Notes:

- N/A

crates/assistant/src/slash_command/prompt_command.rs

@@ -43,12 +43,7 @@ impl SlashCommand for PromptSlashCommand {
                 .prompts()
                 .into_iter()
                 .enumerate()
-                .filter_map(|(ix, prompt)| {
-                    prompt
-                        .1
-                        .title()
-                        .map(|title| StringMatchCandidate::new(ix, title.into()))
-                })
+                .map(|(ix, prompt)| StringMatchCandidate::new(ix, prompt.1.title().to_string()))
                 .collect::<Vec<_>>();
             let matches = fuzzy::match_strings(
                 &candidates,
@@ -86,11 +81,10 @@ impl SlashCommand for PromptSlashCommand {
             let prompt = library
                 .prompts()
                 .into_iter()
-                .filter_map(|prompt| prompt.1.title().map(|title| (title, prompt)))
-                .find(|(t, _)| t == &title)
+                .find(|prompt| &prompt.1.title().to_string() == &title)
                 .with_context(|| format!("no prompt found with title {:?}", title))?
                 .1;
-            Ok(prompt.1.content().to_owned())
+            Ok(prompt.body())
         });
         SlashCommandInvocation {
             output,