fix(gateway): reject oversized sparse array total_length with InvalidArgument

Guard against proto uint32 total_length values that exceed Array.MaxLength
before casting; the previous checked cast threw OverflowException (gRPC
Internal) instead of the intended InvalidArgument. Adds tests for the new
guard, for the null-value ArgumentNullException path, and removes the
checked keyword (redundant after the guard).

src/ZB.MOM.WW.MxGateway.Server/Sessions/SparseArrayExpander.cs

@@ -62,7 +62,13 @@ internal static class SparseArrayExpander
             throw Invalid($"Sparse array element_data_type '{elementType}' is not a supported scalar element type.");
         }
 
-        int length = checked((int)totalLength);
+        if (totalLength > (uint)Array.MaxLength)
+        {
+            throw Invalid(
+                $"Sparse array total_length {totalLength} exceeds the maximum supported array length {Array.MaxLength}.");
+        }
+
+        int length = (int)totalLength;
         HashSet<uint> seenIndices = new();
 
         foreach (MxSparseElement element in sparse.Elements)

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SparseArrayExpanderTests.cs

@@ -192,4 +192,19 @@ public sealed class SparseArrayExpanderTests
         Assert.Equal(MxValue.KindOneofCase.Int32Value, value.KindCase);
         Assert.Equal(42, value.Int32Value);
     }
+
+    [Fact]
+    public void Expand_NullValue_ThrowsArgumentNull()
+    {
+        Assert.Throws<ArgumentNullException>(() => SparseArrayExpander.Expand(null!));
+    }
+
+    [Fact]
+    public void Expand_TotalLengthExceedsMaxArrayLength_Throws()
+    {
+        MxValue value = SparseValue(MxDataType.Integer, 2_147_483_648u);
+
+        RpcException ex = Assert.Throws<RpcException>(() => SparseArrayExpander.Expand(value));
+        Assert.Equal(StatusCode.InvalidArgument, ex.StatusCode);
+    }
 }