fix(extmark): builtin completion can still affect nearby extmarks #31387

Problem:
Built-in completion can still affect nearby extmarks. #31384

Solution:
Restore extmarks when completion leader changes.

(cherry picked from commit c7ec010ade)

test/functional/editor/completion_spec.lua

@@ -1188,5 +1188,15 @@ describe('completion', function()
       aaaaa                                                       |
       {5:-- INSERT --}                                                |
     ]])
+    -- Also when completion leader is changed #31384
+    feed('<Esc>hi<C-N><C-P>a')
+    screen:expect({
+      grid = [[
+        {9:aa}a^aa                                                       |
+        {4:aaaa           }                                             |
+        {4:aaaaa          }                                             |
+        {5:-- Keyword completion (^N^P) }{19:Back at original}               |
+      ]],
+    })
   end)
 end)