Add reminder message about system prompt (#28344)

Trying out sending the model a reminder message about code blocks in the
system prompt. If this seems to work well, we can include more specific
reminder messages, e.g. tool-specific ones.

Release Notes:

- N/A

crates/prompt_store/src/prompts.rs

@@ -261,6 +261,12 @@ impl PromptBuilder {
             .render("assistant_system_prompt", context)
     }
 
+    pub fn generate_assistant_system_prompt_reminder(&self) -> Result<String, RenderError> {
+        self.handlebars
+            .lock()
+            .render("assistant_system_prompt_reminder", &())
+    }
+
     pub fn generate_inline_transformation_prompt(
         &self,
         user_prompt: String,