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/Value.cs

@@ -0,0 +1,28 @@
+using System.Text.Json.Serialization;
+
+namespace Strata.SqlTools.Query;
+
+public class Value
+{
+    public string ColumnAlias { get; }
+    public string Calculation { get; }
+    public IEnumerable<int> CalculationDataColumnIds { get; }
+    public IEnumerable<string> AliasedIds { get; }
+
+    public IEnumerable<CalculationFilterGroup> FilterGroups { get; }
+
+    public Value() : this(string.Empty, string.Empty, new int[0], new string[0], new CalculationFilterGroup[0])
+    {
+        // FilterGroups = new List<CalculationFilterGroup>();
+    }
+
+    [JsonConstructor]
+    public Value(string columnAlias, string calculation, IEnumerable<int> calculationDataColumnIds, IEnumerable<string> aliasedIds, IEnumerable<CalculationFilterGroup> filterGroups)
+    {
+        ColumnAlias = columnAlias;
+        Calculation = calculation;
+        CalculationDataColumnIds = calculationDataColumnIds ?? Array.Empty<int>();
+        AliasedIds = aliasedIds ?? aliasedIds ?? Array.Empty<string>();
+        FilterGroups = filterGroups?.Where(x => x.IsValid()).ToList() ?? new List<CalculationFilterGroup>();
+    }
+}