feat(datasync): add JsonColumnSchema record for ETL column metadata

NEW/src/JdeScoping.DataSync/Etl/Models/JsonColumnSchema.cs

@@ -0,0 +1,25 @@
+namespace JdeScoping.DataSync.Etl.Models;
+
+/// <summary>
+/// Defines a column schema for JSON-to-DataReader mapping.
+/// </summary>
+public record JsonColumnSchema(
+    string Name,
+    Type ClrType,
+    bool IsNullable = true)
+{
+    /// <summary>
+    /// Gets the SQL type name for this column (used in error messages).
+    /// </summary>
+    public string SqlTypeName => ClrType switch
+    {
+        Type t when t == typeof(string) => "VARCHAR",
+        Type t when t == typeof(int) => "INT",
+        Type t when t == typeof(long) => "BIGINT",
+        Type t when t == typeof(decimal) => "DECIMAL",
+        Type t when t == typeof(DateTime) => "DATETIME2",
+        Type t when t == typeof(bool) => "BIT",
+        Type t when t == typeof(byte[]) => "VARBINARY",
+        _ => "UNKNOWN"
+    };
+}