feat(05-01): make helper methods internal and add unit tests

- Changed IsThrottleException to internal static in ExecuteQueryRetryHelper
- Changed BuildPagedViewXml to internal static in SharePointPaginationHelper
- Created ExecuteQueryRetryHelperTests: 5 tests (throttle true x3, non-throttle false, nested false)
- Created SharePointPaginationHelperTests: 5 tests (null, empty, whitespace, replace, append)

SharepointToolbox.Tests/Helpers/ExecuteQueryRetryHelperTests.cs

@@ -0,0 +1,33 @@
+using SharepointToolbox.Core.Helpers;
+
+namespace SharepointToolbox.Tests.Helpers;
+
+[Trait("Category", "Unit")]
+public class ExecuteQueryRetryHelperTests
+{
+    [Theory]
+    [InlineData("The request has been throttled -- 429")]
+    [InlineData("Service unavailable 503")]
+    [InlineData("SharePoint has throttled your request")]
+    public void IsThrottleException_ThrottleMessages_ReturnsTrue(string message)
+    {
+        var ex = new Exception(message);
+        Assert.True(ExecuteQueryRetryHelper.IsThrottleException(ex));
+    }
+
+    [Fact]
+    public void IsThrottleException_NonThrottleMessage_ReturnsFalse()
+    {
+        var ex = new Exception("File not found");
+        Assert.False(ExecuteQueryRetryHelper.IsThrottleException(ex));
+    }
+
+    [Fact]
+    public void IsThrottleException_NestedThrottleInInnerException_ReturnsFalse()
+    {
+        // Documents current behavior: only top-level Message is checked.
+        // Inner exceptions with "429" are NOT currently detected.
+        var ex = new Exception("outer", new Exception("429"));
+        Assert.False(ExecuteQueryRetryHelper.IsThrottleException(ex));
+    }
+}