fix: sort folds by from value for deterministic output order

pairs() iteration order is non-deterministic across LuaJIT builds,
causing the merged fold list order to vary. Sort by from value so the
output is always predictable.

Also fix the merge test to use was_called_with with the sorted order.

Author: CnsMaple

lua/opencode/ui/renderer/buffer.lua

@@ -699,6 +699,7 @@ function M.update_part_folds(part_id)
       table.insert(new_global, f)
     end
   end
+  table.sort(new_global, function(a, b) return a.from < b.from end)
   ctx.global_folds = new_global
   output_window.set_folds(new_global)
 end

tests/unit/renderer_buffer_spec.lua

@@ -183,7 +183,6 @@ describe('update_part_folds', function()
 
     buffer.update_part_folds('part_a')
 
-    assert.stub(set_folds_stub).was_called()
     assert.stub(set_folds_stub).was_called_with({
       { from = 5, to = 8 },
       { from = 10, to = 13 },