[RF] Improve `RooHistPdf::plotSamplingHint`

In the implementation of `RooHistPdf::plotSamplingHint`, some of the
returned values can be outside the variable definition range. This might
lead to invalid evaluations when the sampling hints are also used to
evaluate other PDF components of a composite PDF, as reported in #11482.

This commit suggests an improved algorithm for the sampling hint:
consider only bin boundaries that are strictly between the plot limits
within some epsilon, and for the limits just sample two additional
points left and light that are some epsilon aways from the plot limits.

The `stressRooFit_ref.root` reference file needs to be updated with this
change, because the sampling changes the RooPlots that are used as
reference objects.

Closes #11482.