Reject BattleSnake submissions missing a server entrypoint

validate_code was a pure function-def check, so a main.py that drops the
'if __name__ == "__main__": run_server(...)' block passed validation but
failed to start at runtime (forfeit). Require the entrypoint statically.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: john-b-yang

codeclash/arenas/battlesnake/battlesnake.py

@@ -203,4 +203,11 @@ def validate_code(self, agent: Player) -> tuple[bool, str | None]:
                 error_msg.append(f"There should be a `{func}` function implemented in `{self.submission}`")
         if len(error_msg) > 0:
             return False, "\n".join(error_msg + ["Don't change the function signatures!"])
+        
+	# `python main.py` boots the server via the __main__ entrypoint.
+        if "__main__" not in bot_content or "run_server" not in bot_content:
+            return False, (
+                f"`{self.submission}` must keep its server entrypoint so the bot starts: the "
+                '`if __name__ == "__main__": run_server(...)` block at the bottom of the file.'
+            )
         return True, None