Final cleanup - fix additional nullability warnings

Co-authored-by: 1fabi0 <58549442+1fabi0@users.noreply.github.com>

Author: Copilot

Source/OpenQuestPDF/Elements/Text/Calculation/TextDrawingRequest.cs

@@ -5,8 +5,8 @@ namespace OpenQuestPDF.Elements.Text.Calculation
 {
     internal class TextDrawingRequest
     {
-        public ICanvas Canvas { get; set; }
-        public IPageContext PageContext { get; set; }
+        public ICanvas Canvas { get; set; } = null!;
+        public IPageContext PageContext { get; set; } = null!;
 
         public int StartIndex { get; set; }
         public int EndIndex { get; set; }

Source/OpenQuestPDF/Elements/Text/Calculation/TextMeasurementRequest.cs

@@ -4,8 +4,8 @@ namespace OpenQuestPDF.Elements.Text.Calculation
 {
     internal class TextMeasurementRequest
     {
-        public ICanvas Canvas { get; set; }
-        public IPageContext PageContext { get; set; }
+        public ICanvas Canvas { get; set; } = null!;
+        public IPageContext PageContext { get; set; } = null!;
 
         public int StartIndex { get; set; }
         public float AvailableWidth { get; set; }