Flaky tests/distributions/test_timeseries.py::TestGARCH11::test_batched_size

### Description

Failed last time in https://github.com/pymc-devs/pymc/actions/runs/13036553157/job/36368381129?pr=7664#step:5:646 but I've seen it multiple times before.

We should fix it, either increase tolerance or make smaller sequences