chore(sonar): cache JsonSerializerOptions in test fixture (CA1869)

Promote the `new JsonSerializerOptions { Converters = { ... } }` instance
that `OneTimeSetup` was constructing on each fixture run to a
`private static readonly JsonSerializerOptions _jsonOptions` field, so
the converter list isn't rebuilt per fixture. Strictly cosmetic here
(OneTimeSetup runs once) but it's the change the analyzer wants and the
field is the more idiomatic JsonSerializer pattern.

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

tests/Strata.SqlTools.Rules.Tests/UnitTest1.cs

@@ -11,20 +11,22 @@ public class Tests
     private static readonly Guid _pesDataTableGUID = new Guid("41639c8f-fecf-4449-b6e6-53f796c0c3e4");
     private const string EncounterIDKey = "DimPatientEnEncounterID";
 
+    private static readonly JsonSerializerOptions _jsonOptions = new()
+    {
+        Converters =
+        {
+            new HierarchicalDataConverter(),
+            new FlatDataConverter(),
+            new ObjectToInferredTypesConverter()
+        }
+    };
+
     public Dictionary<long, IHierarchicalData>? _encounterLookup;
 
     [OneTimeSetUp]
     public void OneTimeSetup()
     {
-        var allData = JsonSerializer.Deserialize<List<IHierarchicalData>>(File.ReadAllText(@"Data.json"), new JsonSerializerOptions
-        {
-            Converters =
-            {
-                new HierarchicalDataConverter(),
-                new FlatDataConverter(),
-                new ObjectToInferredTypesConverter()
-            }
-        });
+        var allData = JsonSerializer.Deserialize<List<IHierarchicalData>>(File.ReadAllText(@"Data.json"), _jsonOptions);
 
         if (allData == null || allData.Count == 0)
         {