I do not think parenthesis requires escaping, so would try without the escaping and see if the error disappears or changes.
You are querying a filed mapped as text, which by default uses the standard analyzer. I believe this removes parentesis from the indexed text so you will not be able to run your query against that field. You can use the analyze API to see how the contant and query string are tokenized, which will help you troubleshoot this type of issues.
The source_title.keyword sub field will contain the full string indexed as one component (as long as it is no longer than 256 characters. You should be able to query this field if they are not too long, but will probably need to use a wildcard query, which can be very slow.