fix(on_result_diagnostic): treat end_lnum < 1 as unitialized

The end of the diagnostic range was previously calculated as `item.end_lnum and item.end_lnum - 1`. Given `item = { lnum: 95, end_lnum: 0, ... }`, this calculation would span everything from the beginning up to line 95. I believe that an `end_lnum` value of zero should be treated as uninitialized (because lines are 1-indexed). Therefore, I think the item should be considered a single-line item.

lua/overseer/component/on_result_diagnostics.lua

@@ -68,7 +68,7 @@ local comp = {
               message = item.text,
               severity = type_to_severity[item.type] or vim.diagnostic.severity.ERROR,
               lnum = (item.lnum or 1) - 1,
-              end_lnum = item.end_lnum and (item.end_lnum - 1),
+              end_lnum = (item.end_lnum and item.end_lnum > 0) and (item.end_lnum - 1) or item.lnum,
               col = item.col or 0,
               end_col = item.end_col,
               source = task.name,