Fix insert bounding boxes for rotated blocks

[chui-xelleron]

Insert.GetBoundingBox() now transforms all eight corners of the block bounding box instead of only the min/max pair. That avoids underestimating the extents when the inserted block is rotated or mirrored.

This keeps the change small and targeted:

• empty/infinite boxes are passed through unchanged
• the regression test uses a rotated rectangular solid to prove the old min/max shortcut was wrong

Verified with:

• dotnet test src/ACadSharp.Tests/ACadSharp.Tests.csproj -f net9.0 --filter FullyQualifiedName~ACadSharp.Tests.Entities.InsertTest.GetBoundingBoxUsesAllTransformedCorners -v minimal