fix(diagnostic): stack _tags hl-groups in a single extmark #38654

Problem:
Diagnostic highlight groups were applied by iterating and calling
`vim.hl.range` for each group individually. That resulted in multiple
extmarks with the same priority being created separately, which does not
allow `DiagnosticUnnecessary` and `DiagnosticDeprecated` with matching
options override `Diagnostic*` styling.

Solution:
Pass the list of hl-groups to `vim.hl.range` so they are applied
together in the correct order.

test/functional/lua/diagnostic_spec.lua

@@ -1,5 +1,6 @@
 local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
+local Screen = require('test.functional.ui.screen')
 
 local command = n.command
 local clear = n.clear
@@ -2041,29 +2042,41 @@ describe('vim.diagnostic', function()
     it(
       'shows deprecated and unnecessary highlights in addition to severity-based highlights',
       function()
-        ---@type string[]
-        local result = exec_lua(function()
-          local diagnostic = _G.make_error('Some error', 0, 0, 0, 0, 'source x')
+        local screen = Screen.new(50, 3)
+        screen:set_default_attr_ids({
+          --- DiagnosticUnderlineError + DiagnosticUnnecessary + DiagnosticDeprecated combined
+          [1] = {
+            background = Screen.colors.Red1,
+            strikethrough = true,
+            underline = true,
+            special = Screen.colors.Red1,
+            bold = true,
+          },
+        })
+
+        command('hi DiagnosticUnderlineError guibg=Red gui=underline guisp=Red')
+        command('hi DiagnosticUnnecessary gui=bold')
+        command('hi DiagnosticDeprecated gui=strikethrough')
+
+        exec_lua(function()
+          vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+          vim.diagnostic.config({
+            signs = false,
+          })
+
+          local diagnostic = _G.make_error('Some error', 0, 0, 0, 3, 'source x')
           diagnostic._tags = {
             deprecated = true,
             unnecessary = true,
           }
-
-          local diagnostics = { diagnostic }
-          vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, diagnostics)
-
-          local extmarks = _G.get_underline_extmarks(_G.diagnostic_ns)
-          local hl_groups = vim.tbl_map(function(extmark)
-            return extmark[4].hl_group
-          end, extmarks)
-          return hl_groups
+          vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, { diagnostic })
         end)
 
-        eq({
-          'DiagnosticDeprecated',
-          'DiagnosticUnnecessary',
-          'DiagnosticUnderlineError',
-        }, result)
+        screen:expect([[
+          {1:^1st} line of text                                  |
+          2nd line of text                                  |
+                                                            |
+        ]])
       end
     )