feat: add IsNull and IsNotNull unary condition operators

[AmirTahan80]

Summary

Adds first-class unary null-checking operators (IsNull and IsNotNull) across the entire QueryForge ecosystem.

Callers can now express IS NULL and IS NOT NULL conditions explicitly using dedicated operators without having to pass
ull as a value argument to Equals / NotEquals.

• ConditionOperator.IsNull (11): Evaluates to true when the column/property is null.
• ConditionOperator.IsNotNull (12): Evaluates to true when the column/property is not null.

Changes

• Core (PepperX.QueryForge):

  • Added IsNull (11) and IsNotNull (12) to ConditionOperator enum.
  • Added ConditionSemantics.IsUnaryOperator helper.
  • Updated ConditionSemantics.IsExecutable to allow unary operators to execute without requiring a Value.
  • Implemented evaluation in InMemoryQueryEngine with null-safe three-valued logic.

• Dapper (PepperX.QueryForge.Dapper):

  • Added IS NULL and IS NOT NULL SQL fragment generation in SqlQueryCompiler.BuildCondition without allocating unnecessary query parameters.

• EF Core (PepperX.QueryForge.EFCore):

  • Added expression compilation for IsNull and IsNotNull in ExpressionCompiler.BuildCondition.
  • Added inversion logic in BuildNegatedCondition so negations (NOT (col IS NULL)) cleanly invert without invalid EF Core null compensation.

• Conformance Tests (PepperX.QueryForge.Conformance):

  • Added conformance tests covering nullable columns, non-nullable columns, and negated condition groups.

• Sample API (PepperX.QueryForge.Sample.WebApi):

  • Updated Requests.http with operator documentation and sample requests # 35a and # 35b.

Verification

All 195 unit and integration tests passing (dotnet test).