refactor: Handle KeyboardInterrupt with user-assistant message pair and add env check for sanity_check_messages

aider/coders/base_coder.py

@@ -1359,8 +1359,10 @@ class Coder:
                 interrupted = True
 
         if interrupted:
-            content += "\n^C KeyboardInterrupt"
-            self.cur_messages += [dict(role="assistant", content=content)]
+            self.cur_messages += [
+                dict(role="user", content="^C KeyboardInterrupt"),
+                dict(role="assistant", content="I see that you interrupted my previous reply."),
+            ]
             return
 
         edited = self.apply_updates()

aider/sendchat.py

@@ -1,5 +1,6 @@
 import hashlib
 import json
+import os
 import time
 
 from aider.dump import dump  # noqa: F401
@@ -51,7 +52,8 @@ def send_completion(
 ):
     #
     #
-    # sanity_check_messages(messages)
+    if os.environ.get("AIDER_SANITY_CHECK_TURNS"):
+        sanity_check_messages(messages)
     #
     #