feat: add Output field to FunctionCall struct

- Added the `Output` field to the `FunctionCall` struct as described in the OpenAI API docs.
- This field is optional and is of type `*string`.

chat.go

@@ -187,7 +187,8 @@ type ToolCall struct {
 type FunctionCall struct {
 	Name string `json:"name,omitempty"`
 	// call function with arguments in JSON format
-	Arguments string `json:"arguments,omitempty"`
+	Arguments string  `json:"arguments,omitempty"`
+	Output    *string `json:"output,omitempty"`
 }
 
 type ChatCompletionResponseFormatType string

run_test.go

@@ -22,6 +22,7 @@ func TestRun(t *testing.T) {
 	order := "desc"
 	after := "asst_abc122"
 	before := "asst_abc124"
+	functionCallOutput := "test"
 
 	client, server, teardown := setupOpenAITestServer()
 	defer teardown()
@@ -52,6 +53,18 @@ func TestRun(t *testing.T) {
 							Object:    "run",
 							CreatedAt: 1234567890,
 							Status:    openai.RunStepStatusCompleted,
+							StepDetails: openai.StepDetails{
+								Type: openai.RunStepTypeToolCalls,
+								ToolCalls: []openai.ToolCall{
+									{
+										Function: openai.FunctionCall{
+											Name:      "test",
+											Arguments: "{}",
+											Output:    &functionCallOutput,
+										},
+									},
+								},
+							},
 						},
 					},
 				})