fix(completion): wrong CompleteDone reason for auto-inserted sole match #38280

Problem: #38169 used compl_used_match to determine the CompleteDone
reason, but this fires too broadly, it also changes the reason to
"accept" when the popup was shown and the user dismissed it with <Esc>
or <Space>, breaking snippet completion with autocomplete.

Solution: Instead of checking compl_used_match in, check whether the pum
was never shown (compl_match_array == NULL) in ins_compl_stop().
When a match was inserted but the pum never displayed,
set the completed word so CompleteDone fires with reason "accept".
This keeps the "discard" reason intact when the user dismisses a visible
pum without confirming.

test/functional/plugin/lsp/completion_spec.lua

@@ -1001,9 +1001,6 @@ describe('vim.lsp.completion: protocol', function()
         },
       },
     }
-    exec_lua(function()
-      vim.o.completeopt = 'menuone,noselect'
-    end)
     local client_id = create_server('dummy', completion_list)
 
     exec_lua(function()
@@ -1042,9 +1039,6 @@ describe('vim.lsp.completion: protocol', function()
       isIncomplete = false,
       items = { { label = 'hello' } },
     }
-    exec_lua(function()
-      vim.o.completeopt = 'menuone,noselect'
-    end)
     local client_id = create_server('dummy', completion_list, {
       resolve_result = {
         label = 'hello',