refactor(compaction): append fitting input_text parts without copying

BN01 quality finding (Low): _truncate_input_text_parts copied each
fully-fitting part with {**part} although it never mutates it. The
fully-fitting item path already appends the original item uncopied,
and the clipped boundary part is still a new dict. No caller mutates
the returned parts (the result is only placed into the retained
message), so the copy was dead work.

agent/native_compaction.py

@@ -206,7 +206,7 @@ def _truncate_input_text_parts(content: List[Dict[str, Any]], budget: int) -> Li
             if lo:
                 head.append({**part, "text": text[:lo]})
             break
-        head.append({**part})
+        head.append(part)
         budget -= cost
     return head