fix: add explicit tostring() for thread in string.format() for Lua 5.1 compatibility (#1461)

Co-authored-by: lewis6991 <7904185+lewis6991@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

lua/gitsigns/async.lua

@@ -170,7 +170,7 @@ end
 function Task:_traceback(msg, _lvl)
   _lvl = _lvl or 0
 
-  local thread = ('[%s] '):format(self._thread)
+  local thread = ('[%s] '):format(tostring(self._thread))
 
   local child = self._current_child
   if getmetatable(child) == Task then