Implement missing filter operators (CONTAINS, IN, EXISTS, IS_NULL)

[kamiazya]

Background

During the Context View PR review (#165), it was identified that the ContextViewFilter documentation mentions operators that aren't implemented in the parser/AST.

Problem

The following operators are documented but not implemented:

• CONTAINS
• IN
• EXISTS
• IS_NULL

Additionally, factory methods in ContextViewFilter generate syntax that the parser cannot parse.

Solution

Either:

1. Implement full support for these operators in:
   • FilterToken (add new token types)
   • FilterExpressionAST (add new node types)
   • FilterExpressionParser (parse the syntax)
   • FilterEvaluationService (evaluate the operators)
2. Or remove the documentation and factory methods until support is added

References

• PR feat: implement Context View functionality for filtered scope management #165 review comments about documented but unsupported operators