[JS] chat.send({messages}) should append, not replace

**To Reproduce**
```ts
ai.chat({
  messages: [{role: "system", content: [{text: "System Message"]}],
}).send({
  messages: [{role: "user", content: [{text: "User Message"}]}]
}),
```

**Expected behavior**
LLM receives both messages.

**Actual Behavior**
LLM receives only user message.