fix(api): leaked error message in nvim_eval_statusline() #41690

Problem:
`find_window_by_handle()` returns NULL only after `VALIDATE_INT` has
already set `err`, so the `api_set_error()` that followed allocated a
second message over the first pointer and leaked it.

Solution:
Drop the redundant call and keep the helper's message, which is already
set and better worded.

AI-assisted

src/nvim/api/vim.c

@@ -2289,7 +2289,6 @@ DictAs(eval_statusline_ret) nvim_eval_statusline(String str, Dict(eval_statuslin
 
   win_T *wp = opts->use_tabline ? curwin : find_window_by_handle(window, err);
   if (wp == NULL) {
-    api_set_error(err, kErrorTypeException, "unknown winid %d", window);
     return result;
   }
 

test/functional/api/vim_spec.lua

@@ -4524,6 +4524,12 @@ describe('API', function()
       }, api.nvim_eval_statusline('%%StatusLineString%#WarningMsg#WithHighlights', {}))
     end)
 
+    it('reports an invalid window once', function()
+      -- find_window_by_handle() already sets the error, so a second
+      -- api_set_error() here would allocate a message over that one and leak it.
+      eq('Invalid window id: 23', pcall_err(api.nvim_eval_statusline, 'a', { winid = 23 }))
+    end)
+
     it("doesn't exceed maxwidth", function()
       eq({
         str = 'Should be trun>',