`FixedNBins` fails in marginal-mode estimation

[BJMCox]

Problem

FixedNBins is available as a binning policy, and BinnedModeEstimator accepts it. Marginal-mode estimation then raises a MethodError. Automatic binning works on the same samples.

Impact

Users cannot select a fixed number of bins for marginal-mode estimation.

Reproduction

using BAT

ctx = BATContext()
samples = DensitySampleVector(
    [[1.0], [2.0], [3.0], [4.0], [5.0]],
    zeros(5),
)

bat_marginalmode(
    samples,
    BinnedModeEstimator(binning = FixedNBins(nbins = 2)),
    ctx,
)

Output

MethodError: no method matching _get_bining_impl(::FixedNBins, ::Int64, ::Float64, ::Vector{Float64})

Expected result

Positive fixed bin counts should work through marginal-mode estimation and select the requested number of bins.

Related to #166.