fix(ui): no fast context for prompt message kinds #31224

Problem:  No longer able to show prompt messages with vim.ui_attach().
Solution: Do not execute callback in fast context for prompt message
          kinds. These events must be safe to show the incoming message
          so the event itself serves to indicate that the message
          should be shown immediately.

test/functional/lua/ui_event_spec.lua

@@ -237,20 +237,21 @@ describe('vim.ui_attach', function()
     })
   end)
 
-  it('aborts :function on error with ext_messages', function()
+  it('msg_show in fast context', function()
     exec_lua([[
     vim.ui_attach(ns, { ext_messages = true }, function(event, _, content)
       if event == "msg_show" then
-        -- "fast-api" does not prevent aborting :function
         vim.api.nvim_get_runtime_file("foo", false)
         -- non-"fast-api" is not allowed in msg_show callback and should be scheduled
         local _, err = pcall(vim.api.nvim_buf_set_lines, 0, -2, -1, false, { content[1][2] })
+        pcall(vim.api.nvim__redraw, { flush = true })
         vim.schedule(function()
           vim.api.nvim_buf_set_lines(0, -2, -1, false, { content[1][2], err })
         end)
       end
     end)
     ]])
+    -- "fast-api" does not prevent aborting :function
     feed(':func Foo()<cr>bar<cr>endf<cr>:func Foo()<cr>')
     screen:expect({
       grid = [[
@@ -267,6 +268,23 @@ describe('vim.ui_attach', function()
         },
       },
     })
+    -- No fast context for prompt message kinds
+    feed(':%s/Function/Replacement/c<cr>')
+    screen:expect({
+      grid = [[
+        ^E122: {10:Function} Foo already exists, add !|
+         to replace it                          |
+        replace with Replacement (y/n/a/q/l/^E/^|
+        Y)?                                     |
+        {1:~                                       }|
+      ]],
+      messages = {
+        {
+          content = { { 'replace with Replacement (y/n/a/q/l/^E/^Y)?', 6, 19 } },
+          kind = 'confirm_sub',
+        },
+      },
+    })
   end)
 
   it('detaches after excessive errors', function()