Rename private static field to satisfy TestApiConsistency

TestApiConsistency.TestPrivateFieldNames (run in the _J-S test shard) scans
the whole Lucene.Net assembly and requires private fields to be camelCase
(optionally _-prefixed) or ALL_CAPS. The new
ReflectionTypeExtensions.LuceneNetNamespaceRegex was PascalCase and failed
that check. Rename it to luceneNetNamespaceRegex (camelCase), matching the
sibling packageMappingCache field. No behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: paulirwin

src/Lucene.Net/Support/Reflection/ReflectionTypeExtensions.cs

@@ -30,7 +30,7 @@ namespace Lucene.Net.Reflection
     [NoLuceneEquivalent]
     public static class ReflectionTypeExtensions
     {
-        private static readonly Regex LuceneNetNamespaceRegex = new("^Lucene.Net.", RegexOptions.Compiled);
+        private static readonly Regex luceneNetNamespaceRegex = new("^Lucene.Net.", RegexOptions.Compiled);
 
         /// <summary>
         /// Gets the <see cref="LuceneTypeInfo"/> for the specified type.
@@ -100,7 +100,7 @@ public static class ReflectionTypeExtensions
             if (packageName == null)
             {
                 // assume all lower-case, any casing will have to be mapped manually
-                packageName = LuceneNetNamespaceRegex.Replace(type.Namespace, "org.apache.lucene.").ToLowerInvariant();
+                packageName = luceneNetNamespaceRegex.Replace(type.Namespace, "org.apache.lucene.").ToLowerInvariant();
             }
 
             var packageNameParts = packageName.Split('.');