fix(lua): close vim.defer_fn() timer if vim.schedule() failed (#37647)

Problem:
Using vim.defer_fn() just before Nvim exit leaks luv handles.

Solution:
Make vim.schedule() return an error message if scheduling failed.
Make vim.defer_fn() close timer if vim.schedule() failed.

runtime/lua/vim/_core/editor.lua

@@ -509,25 +509,28 @@ end
 --- Defers calling {fn} until {timeout} ms passes.
 ---
 --- Use to do a one-shot timer that calls {fn}
---- Note: The {fn} is |vim.schedule_wrap()|ped automatically, so API functions are
+--- Note: The {fn} is |vim.schedule()|d automatically, so API functions are
 --- safe to call.
 ---@param fn function Callback to call once `timeout` expires
 ---@param timeout integer Number of milliseconds to wait before calling `fn`
 ---@return table timer luv timer object
 function vim.defer_fn(fn, timeout)
   vim.validate('fn', fn, 'callable', true)
+
   local timer = assert(vim.uv.new_timer())
-  timer:start(
-    timeout,
-    0,
-    vim.schedule_wrap(function()
+  timer:start(timeout, 0, function()
+    local _, err = vim.schedule(function()
       if not timer:is_closing() then
         timer:close()
       end
 
       fn()
     end)
-  )
+
+    if err then
+      timer:close()
+    end
+  end)
 
   return timer
 end

runtime/lua/vim/_meta/builtin.lua

@@ -178,6 +178,8 @@ function vim.iconv(str, from, to, opts) end
 --- Schedules {fn} to be invoked soon by the main event-loop. Useful
 --- to avoid |textlock| or other temporary restrictions.
 --- @param fn fun()
+--- @return nil result
+--- @return string? err Error message if scheduling failed, `nil` otherwise.
 function vim.schedule(fn) end
 
 --- Waits up to `time` milliseconds, until `callback` returns `true` (success). Executes