fix(diagnostics): diagnostic just after EOL is not highlighted #34085

Fixes #34013

Problem:
when diagnostic is set right after EOL, underline handler looks
inconsistent compared to other handlers, visually underline is shown
starting from the next line. On top of that it's also inconsistent with
open_float and jump.

Solution:
clamp starting column position in underline handler to be right before
EOL to make it visible and consistent with other handlers, open_float
and jump.

runtime/lua/vim/diagnostic.lua

@@ -1581,11 +1581,14 @@ M.handlers.underline = {
         end
       end
 
+      local lines =
+        vim.api.nvim_buf_get_lines(diagnostic.bufnr, diagnostic.lnum, diagnostic.lnum + 1, true)
+
       vim.hl.range(
         bufnr,
         underline_ns,
         higroup,
-        { diagnostic.lnum, diagnostic.col },
+        { diagnostic.lnum, math.min(diagnostic.col, #lines[1] - 1) },
         { diagnostic.end_lnum, diagnostic.end_col },
         { priority = get_priority(diagnostic.severity) }
       )