apacheGH-41198: [C#] Fix concatenation of union arrays (apache#41226)

### Rationale for this change

Fixes concatenation of union arrays.

### What changes are included in this PR?

* Re-enables union array concatenation tests that were disabled in apache#41197 after making union array comparisons more thorough in the `ArrowReaderVerifier`
* Updates the union array concatenation logic to account for array lengths when concatenating the type and offset buffers, and fixes how the base offset is calculated.
* Fixes creating the type buffers for the array concatenation tests.

### Are these changes tested?

Yes, this uses the existing `ArrowArrayConcatenatorTests` tests.

### Are there any user-facing changes?

Yes, this is a user-facing bug fix.
* GitHub Issue: apache#41198

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>

Author: adamreeve

csharp/test/Apache.Arrow.Tests/ArrowArrayConcatenatorTests.cs

@@ -29,12 +29,6 @@ public void TestStandardCases()
         {
             foreach ((List<IArrowArray> testTargetArrayList, IArrowArray expectedArray) in GenerateTestData())
             {
-                if (expectedArray is UnionArray)
-                {
-                    // Union array concatenation is incorrect. See https://github.com/apache/arrow/issues/41198
-                    continue;
-                }
-
                 IArrowArray actualArray = ArrowArrayConcatenator.Concatenate(testTargetArrayList);
                 ArrowReaderVerifier.CompareArrays(expectedArray, actualArray);
             }
@@ -410,15 +404,11 @@ public void Visit(UnionType type)
 
                     for (int j = 0; j < dataList.Count; j++)
                     {
-                        bool includeInResult = IncludeInResult(i, j);
                         byte index = (byte)Math.Min(j % 3, 1);
                         int? intValue = (index == 1) ? dataList[j] : null;
                         string stringValue = (index == 1) ? null : dataList[j]?.ToString();
                         typeBuilder.Append(index);
-                        if (includeInResult)
-                        {
-                            typeResultBuilder.Append(index);
-                        }
+                        typeResultBuilder.Append(index);
 
                         if (isDense)
                         {