fix(chat): prevent panic on UTF-8 boundary when rendering response

When the AI response contains multi-byte characters (e.g. non-ASCII
text adjacent to triple backticks), the byte offset accumulated via
parsed.offset_from() can land in the middle of a UTF-8 character
boundary. The subsequent &buf[offset..] slice then panics at runtime.

Replace the direct slice with buf.get(offset..) which returns None
instead of panicking when the index is out of bounds or misaligned,
and break the loop gracefully in that case.

Fixes #3715

Author: sandikodev

crates/chat-cli/src/cli/chat/mod.rs

@@ -3099,7 +3099,11 @@ impl ChatSession {
 
             // Print the response for normal cases
             loop {
-                let input = Partial::new(&buf[offset..]);
+                // Use `get` to avoid panicking if `offset` lands on a non-UTF-8 boundary,
+                // which can happen when the response contains multi-byte characters (e.g.
+                // non-ASCII text adjacent to triple backticks). See: #3715
+                let Some(slice) = buf.get(offset..) else { break };
+                let input = Partial::new(slice);
                 if self.stdout.should_send_structured_event {
                     match interpret_markdown(input, &mut temp_buf, &mut state) {
                         Ok(parsed) => {