fix(osc52): enable OSC 52 by default in tmux sessions (#26072)

tmux has a set-clipboard option which, when set to 'on', allows
applications to set the system clipboard using the usual OSC 52 escape
sequence.

runtime/plugin/osc52.lua

@@ -2,19 +2,18 @@ local tty = vim.iter(vim.api.nvim_list_uis()):any(function(ui)
   return ui.chan == 1 and ui.stdout_tty
 end)
 
-if
-  not tty
-  or vim.g.clipboard ~= nil
-  or vim.o.clipboard ~= ''
-  or not os.getenv('SSH_TTY')
-  or os.getenv('TMUX')
-then
+if not tty or vim.g.clipboard ~= nil or vim.o.clipboard ~= '' or not os.getenv('SSH_TTY') then
   return
 end
 
 require('vim.termcap').query('Ms', function(cap, seq)
   assert(cap == 'Ms')
 
+  -- Check 'clipboard' and g:clipboard again to avoid a race condition
+  if vim.o.clipboard ~= '' or vim.g.clipboard ~= nil then
+    return
+  end
+
   -- If the terminal reports a sequence other than OSC 52 for the Ms capability
   -- then ignore it. We only support OSC 52 (for now)
   if not seq:match('^\027%]52') then