insights: Use patterntype:literal for all queries unless otherwise defined

Created by: CristinaBirkel

Closes: https://github.com/sourcegraph/sourcegraph/issues/38439

This change is to make sure that we preserve the current query behavior even when the search changes to read /.../ syntax as a regular expression.

Test plan

• backfiller jobs are queued up with the patterntype:literal field, when that field is not specified
• backfiller jobs are queued with whatever patterntype is provided, if one is provided