feat: add In and NotIn membership operators

[AmirTahan80]

Summary

Adds set membership filtering (In and NotIn) across the full QueryForge stack, allowing callers to match or exclude a collection of values in a single condition instead of chaining multiple OR or AND NOT clauses.

• ConditionOperator.In (11): Matches when the column value equals any non-null candidate in the collection.
• ConditionOperator.NotIn (12): Matches when the column value is excluded from all candidates in the collection.
• Empty-list semantics: IN [] matches nothing (0 rows); NOT IN [] matches everything.

Changes

• Core Engine (PepperX.QueryForge):

  • Added In and NotIn to ConditionOperator enum.
  • Extended ConditionSemantics.Unwrap to handle JsonValueKind.Array recursively.
  • Updated ConditionSemantics.IsExecutable for empty-list membership expressions.
  • Implemented null-safe three-valued logic evaluation in InMemoryQueryEngine.

• Dapper Provider (PepperX.QueryForge.Dapper):

  • Implemented BuildMembershipIn with per-candidate parameterization (@p0, @p1, ...) and type coercion.

• EF Core Provider (PepperX.QueryForge.EFCore):

  • Implemented LINQ Enumerable.Contains expression compilation with parameterized typed candidate lists.

• Conformance Tests (PepperX.QueryForge.Conformance):

  • Added 9 comprehensive conformance tests covering all providers and edge cases.

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

  • Added /api/users/filters/membership endpoint with runnable examples in Requests.http.

Verification

All 183 unit and integration tests passing (dotnet test).