assistant: Don't unwrap built-in step resolution prompt (#15704)

This PR removes some `unwrap`s while loading the built-in step
resolution prompt.

While these `unwrap`s are safe today, they are relying on some implicit
contracts that might change in the future. We're using this in a context
where it's easy to propagate an error upwards, so we may as well avoid
the `unwrap`s entirely and use a `Result`.

Release Notes:

- N/A

crates/assistant/src/context.rs

@@ -1407,7 +1407,7 @@ impl Context {
             async move {
                 let prompt_store = cx.update(|cx| PromptStore::global(cx))?.await?;
 
-                let mut prompt = prompt_store.step_resolution_prompt();
+                let mut prompt = prompt_store.step_resolution_prompt()?;
                 prompt.push_str(&step_text);
 
                 request.messages.push(LanguageModelRequestMessage {