Add flatten method and extend filterAnd for nested expressions

[vogievetsky]

This PR adds a new flatten method to SQL expressions and improves the filterAnd method to handle nested AND expressions correctly.

Changes:

1. Added flatten method to SqlExpression:
   - Base implementation in SqlExpression class returns the expression as-is
   - SqlMulti class implementation recursively flattens nested multi expressions with the same operator
   - Allows flattening expressions like a AND (b AND c) to a AND b AND c
2. Enhanced filterAnd method in SqlMulti:
   - Fixed to correctly handle nested AND expressions
   - Now recursively filters through nested expressions instead of just top-level ones
   - Example: a AND (x AND y AND b) with filter removing 'b' now correctly produces a AND (x AND y)
3. Comprehensive test coverage:
   - Added tests for the new flatten method covering various scenarios including deeply nested expressions
   - Added test for nested AND expressions in filterAnd
   - Moved removeColumnFromAnd tests from sql-multi.spec.ts to sql-expression.spec.ts and added invariant tests to ensure correctness

[changeset-bot]

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Latest commit: ac190a7

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