fix(fetch): JSON-stringify NaN and Infinity string data - #3249

With Content-Type: application/json, string data is sent as-is when it already parses as JSON, and JSON-stringified otherwise. is_json_parsable() used json.loads(), which accepts the non-standard constants NaN, Infinity and -Infinity, so those strings went out unquoted as invalid JSON. Node's JSON.parse rejects them, so Node sends "NaN". is_json_parsable() now rejects these constants through parse_constant, and they take the json.dumps() path like any other string.

Follows #3223, which did the same for float data. The existing string-body test now also posts "NaN".

Fixes: microsoft/playwright#43329