fix(chat): skip serializing tool_calls if null, to avoid 422 error

src/v1/chat.rs

@@ -9,6 +9,7 @@ use crate::v1::{common, constants, tool};
 pub struct ChatMessage {
     pub role: ChatMessageRole,
     pub content: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub tool_calls: Option<Vec<tool::ToolCall>>,
 }
 impl ChatMessage {