fix(conversation): return_all_except_first_string drops two messages, not one (kyegomez#2133)

return_all_except_first slices [1:] while return_all_except_first_string
slices [2:], so the string variant silently loses the first real message.
Both document the same contract and history_output_formatter offers them
as sibling output types.

Fixes kyegomez#2112

Author: ayaangazali

swarms/structs/conversation.py

@@ -1364,7 +1364,7 @@ def return_all_except_first_string(self):
         return "\n".join(
             [
                 f"{msg['content']}"
-                for msg in self.conversation_history[2:]
+                for msg in self.conversation_history[1:]
             ]
         )