Ignore scipy.stats._axis_nan_policy.SmallSampleWarning for LaxBackedScipyStatsTests.testMode
It is to fix our CI, the warning itself started occurring on scipy 1.14 due to this change https://github.com/scipy/scipy/pull/20694, which introduced SmallSampleWarning and started emitting it if the input is an empty array (the `a` variable in the randomized parametrized test LaxBackedScipyStatsTests.testMode sometimes happens to be an empty array).
Note, the actual ignored warning is RungimeWarning (the superclass of SmallSampleWarning) to make it backward compatible (scipy.stats._axis_nan_policy.SmallSampleWarning does not exist in scipy prior 1.14, not to mention it being under private declared in a private (_axis_nan_policy) namespace.
PiperOrigin-RevId: 677629866