fix(float): don't unload 'hidden' float buffer with :close! (#39304)

Problem:  When closing floating windows to close a tabpage, if the current
          buffer will unload, buffers contained in those floating windows
          will too (unexpectedly).
Solution: Don't pass along "free_buf" argument; check 'bufhidden' for
          the buffer in the to be closed float.
(cherry picked from commit 5b0ad4a060)

Co-authored-by: luukvbaal <luukvbaal@gmail.com>

test/functional/ui/float_spec.lua

@@ -1072,6 +1072,15 @@ describe('float window', function()
       command('close')
       assert_alive()
     end)
+
+    it('does not unload bufhidden=hide buffer', function()
+      local buf = api.nvim_create_buf(false, true)
+      command('set nohidden')
+      api.nvim_open_tabpage(0, true, {})
+      api.nvim_open_win(buf, false, { relative = 'editor', width = 1, height = 1, row = 0, col = 0 })
+      command('close!')
+      eq(true, api.nvim_buf_is_loaded(buf))
+    end)
   end)
 
   it('placed relative to tabline and laststatus', function()