`chat()` ends up with a different default value for `echo` than direct chat functions

```r
library(ellmer)

chat("openai")$chat("Hi there") # doesn't stream (echo="none")
chat_openai()$chat("Hi there") # streams (echo="output")
```

This happens because the default value for `echo` is decided according to where the function is called. Because we call `chat_openai()` _inside_ `chat()` we lose the default value.