Base class for all filter expressions.
All filter types extend this class and implement the toString() method
to generate the appropriate RediSearch query syntax.
class FilterExpressionDiscriminator property for type-safe filter identification. Each filter type has a unique filterType value.
Combine this filter with another using AND logic.
In RediSearch, AND operations are represented by space-separated conditions
within parentheses: (condition1 condition2)
Combine this filter with another using OR logic.
In RediSearch, OR operations are represented by pipe-separated conditions
within parentheses: (condition1|condition2)
Converts the filter expression to a RediSearch query string.