fix(api): nvim__complete_set requires completeopt=popup #31177

Problem:
If completeopt does not include "popup" flag, nvim__complete_set still
auto-creates a floating preview window.

Solution:
Fail if completeopt does not include the "popup" flag.

test/functional/editor/completion_spec.lua

@@ -10,6 +10,7 @@ local fn = n.fn
 local command = n.command
 local api = n.api
 local poke_eventloop = n.poke_eventloop
+local exec_lua = n.exec_lua
 
 describe('completion', function()
   local screen
@@ -1326,4 +1327,29 @@ describe('completion', function()
       ]],
     })
   end)
+
+  describe('nvim__complete_set', function()
+    it("fails when 'completeopt' does not include popup", function()
+      exec_lua([[
+        function _G.omni_test(findstart, base)
+          if findstart == 1 then
+            return vim.fn.col('.') - 1
+          end
+          return { { word = 'one' } }
+        end
+        vim.api.nvim_create_autocmd('CompleteChanged', {
+          callback = function()
+            local ok, err = pcall(vim.api.nvim__complete_set, 0, { info = '1info' })
+            if not ok then
+              vim.g.err_msg = err
+            end
+          end,
+        })
+        vim.opt.completeopt = 'menu,menuone'
+        vim.opt.omnifunc = 'v:lua.omni_test'
+      ]])
+      feed('S<C-X><C-O>')
+      eq('completeopt option does not include popup', api.nvim_get_var('err_msg'))
+    end)
+  end)
 end)