fix(vulnfeeds): prevent duplicate events in database_specific

[jess-lowe]

combine-to-osv merges database_specific fields using MergeDatabaseSpecificValues from the conversion package. However, the helper function deduplicateList only checked comparable basic types (strings, ints, floats, bools). Non-comparable types (like maps representing JSON objects) were always appended without deduplication.

I updated deduplicateList to check if a type is comparable. If it's not (e.g. a map or slice), it now checks for uniqueness against previously observed uncomparable elements using reflect.DeepEqual.