agents: fix the function call of openai llm.

According to the [guides](https://platform.openai.com/docs/guides/function-calling) of openai function call,
`Once you've executed these function calls in your application, you can provide the result back to the model
by adding one new message to the conversation for each function call, each containing the result of one
function call, with a tool_call_id referencing the id from tool_calls`.
We need to provide the function call result back to the model in line with the specified structure,
otherwise the openai server will verify the parameters and return a 400 status code.

agents/executor_test.go

@@ -2,7 +2,9 @@ package agents_test
 
 import (
 	"context"
+	"fmt"
 	"os"
+	"slices"
 	"strings"
 	"testing"
 
@@ -133,11 +135,12 @@ func TestExecutorWithOpenAIFunctionAgent(t *testing.T) {
 	)
 
 	e := agents.NewExecutor(a)
-	require.NoError(t, err)
 
 	result, err := chains.Run(context.Background(), e, "what is HK singer Eason Chan's years old?") //nolint:lll
 	require.NoError(t, err)
 
-	require.True(t, strings.Contains(result, "47") || strings.Contains(result, "49"),
-		"correct answer 47 or 49 not in response")
+	correctAnswerList := []string{"47", "49", "50"}
+	require.True(t, slices.ContainsFunc(correctAnswerList, func(s string) bool {
+		return strings.Contains(result, s)
+	}), fmt.Sprintf("correct answer %s not in response", correctAnswerList))
 }

agents/openai_functions_agent.go

@@ -180,8 +180,20 @@ func (o *OpenAIFunctionsAgent) constructScratchPad(steps []schema.AgentStep) []l
 
 	messages := make([]llms.ChatMessage, 0)
 	for _, step := range steps {
-		messages = append(messages, llms.FunctionChatMessage{
-			Name:    step.Action.Tool,
+		messages = append(messages, llms.AIChatMessage{
+			ToolCalls: []llms.ToolCall{
+				{
+					ID:   step.Action.ToolID,
+					Type: "function",
+					FunctionCall: &llms.FunctionCall{
+						Name:      step.Action.Tool,
+						Arguments: step.Action.ToolInput,
+					},
+				},
+			},
+		})
+		messages = append(messages, llms.ToolChatMessage{
+			ID:      step.Action.ToolID,
 			Content: step.Observation,
 		})
 	}