[prompts] Better user_instructions handling (#1836)

## Summary
Our recent change in #1737 can sometimes lead to the model confusing
AGENTS.md context as part of the message. But a little prompting and
formatting can help fix this!

## Testing
- Ran locally with a few different prompts to verify the model
behaves well.
- Updated unit tests

codex-rs/core/src/client.rs

@@ -144,11 +144,11 @@ impl ModelClient {
         };
 
         let mut input_with_instructions = Vec::with_capacity(prompt.input.len() + 1);
-        if let Some(ui) = &prompt.user_instructions {
+        if let Some(ui) = prompt.get_formatted_user_instructions() {
             input_with_instructions.push(ResponseItem::Message {
                 id: None,
                 role: "user".to_string(),
-                content: vec![ContentItem::InputText { text: ui.clone() }],
+                content: vec![ContentItem::InputText { text: ui }],
             });
         }
         input_with_instructions.extend(prompt.input.clone());