Fix parsing of inf strings

[ysimonson]

Server events MaxResponseOutputTokens will currently not parse:

called `Result::unwrap()` on an `Err` value: JSON decode error: data did not match any variant of untagged enum MaxResponseOutputTokens

This fixes the issue, though it's a hack as it doesn't validate that the string is inf. I'm not sure how to do that in serde without writing a custom deserializer/visitor.