Support Attachments in MessageRequest (#890)

* add attachments in MessageRequest

* Move tools const to message

* remove const, just use assistanttool const

messages.go

@@ -56,6 +56,7 @@ type MessageRequest struct {
 	Content     string             `json:"content"`
 	FileIds     []string           `json:"file_ids,omitempty"` //nolint:revive // backwards-compatibility
 	Metadata    map[string]any     `json:"metadata,omitempty"`
+	Attachments []ThreadAttachment `json:"attachments,omitempty"`
 }
 
 type MessageFile struct {