refactor(query): extract shared Strata.SqlTools.Query model project

Moves the byte-identical 19-file ExpressionFactory/Query tree (duplicated
across Snowflake and SqlServer) into the new Strata.SqlTools.Query project
under the flat namespace Strata.SqlTools.Query. Both dialect projects now
reference the shared project; the Snowflake copies are deleted.

Also folds in the IDE0028 fix on CalculationFilterGroup.GetValidFilters
(collection expression []), which resolves both new-code IDE0028 smells
in one place now that there is a single copy.

Eliminates the 63 new duplicate lines flagged on the PR and removes the
largest contributor to the project's 11.1% duplication density. No
behavioral change: the moved types are identical to the originals.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Strata.SqlTools.Query/FilterGroup.cs

@@ -0,0 +1,29 @@
+using System.Text.Json.Serialization;
+
+namespace Strata.SqlTools.Query;
+
+public class FilterGroup
+{
+    // Hereditary logical operation applied to all Filters
+    public LogicalOperator LogicalOperator { get; set; }
+
+    public IEnumerable<Filter> Filters { get; }
+
+    public FilterGroup()
+    {
+        LogicalOperator = LogicalOperator.And;
+        Filters = new List<Filter>();
+    }
+
+    [JsonConstructor]
+    public FilterGroup(IEnumerable<Filter> filters, LogicalOperator logicalOperator)
+    {
+        Filters = filters.Where(x => x.IsValid()).ToList();
+        LogicalOperator = logicalOperator;
+    }
+
+    public bool IsValid()
+    {
+        return Filters != null && Filters.Any();
+    }
+}