Fix exception when updating drawing hyperlinks and fix ignored tooltips

[lievendf]

Summary

This PR fixes two bugs in ExcelDrawing.cs when working with hyperlinks on drawings (shapes, pictures, etc.):

1. An exception (NullReferenceException or InvalidOperationException) when changing an existing hyperlink to a new one.
2. Tooltips not being saved when assigning a new hyperlink with a tooltip.

What was wrong?

1. The Exception:
   When you changed a drawing's hyperlink, the setter tried to delete the old link using ZipPackage.DeletePart(...). However, drawing hyperlinks are just XML relationships (.rels), not actual physical files inside the ZIP package. This caused an exception:

   • If the old link was a worksheet reference, it threw a NullReferenceException (because the target URI is null).
   • If the old link was an external web URL, it threw InvalidOperationException: Part does not exist.

2. The Tooltip Bug:
   When writing the tooltip to the drawing's XML, the code checked if (Hyperlink is ExcelHyperLink excelLink). At this point, the backing field hadn't been updated yet, so Hyperlink returned the old hyperlink instead of the new one. This meant any new tooltip you set was ignored.

How this PR fixes it

1. For the Exception: Changed the code to delete the relationship from the drawing part instead of trying to delete a zip package part:

   _drawings.Part.DeleteRelationship(HypRel.Id);

2. For the Tooltip: Changed the check to use the incoming value instead of the old Hyperlink property:

   if (value is ExcelHyperLink excelLink)

Added Tests

Added a unit test DrawingHyperlinkUpdate_ShouldNotThrowException in HyperlinkTest.cs that:

• Assigns an initial external link.
• Updates it to a new external link.
• Updates it to an internal sheet reference.
• Saves the file and reads it back to make sure the tooltip and hyperlink are correct.