feat(opl): expose TextBlock() on IPropertyList

OplTextBlock.TextBlock() was only on the concrete ObjectPropertyList, so
GetProperties(IPropertyList) overrides in content could not reach the
multi-line builder (its constructor is internal, and the interface only
exposed the lower-level AddChunked). Declare TextBlock() on IPropertyList
so content can write `using var block = list.TextBlock();` directly.

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

Projects/Server/PropertyList/IPropertyList.cs

@@ -38,6 +38,9 @@ public interface IPropertyList : ISelfInterpolatedStringHandler
     /** Emits newline-joined text across multiple properties so none exceeds the legacy client's per-property buffer. */
     public void AddChunked(ReadOnlySpan<char> text);
 
+    /** Builder for variable-length multi-line free text; flushes via AddChunked on dispose. */
+    public OplTextBlock TextBlock();
+
     /** Convenience method for $"{value}". */
     public void Add(int number, int value);