BUG: reject negative dispersion in power_equivalence_neginb_2indep - #10426

The checks added in GH-10379, GH-10386, GH-10388, GH-10392, GH-10393,
GH-10399, GH-10405, GH-10406, GH-10411, GH-10420, GH-10421, GH-10423,
GH-10425, GH-10426 and GH-10431 compare the arguments with scalars,
for example `if not 0 < alpha < 1`, `if low > upp` or `if dispersion < 0`.
These raise "The truth value of an array with more than one element is
ambiguous" for arrays and Series. The functions broadcast over these
arguments and return the same values as the calls for the elements in
0.15.0, for example a confidence interval for several alpha, the power
for several equivalence margins or a table of sample sizes.

Test with np.all and the comparison ufuncs instead, which work for
scalars, lists, arrays and Series, and which also reject NaN. The
error messages are unchanged. A new test checks for 50 arguments that an
array of two valid values gives the same result as the two calls with the
scalars, and that one invalid element (including NaN) is rejected.

Follow-up to GH-10379, GH-10386, GH-10388, GH-10392, GH-10393, GH-10399,
GH-10405, GH-10406, GH-10411, GH-10420, GH-10421, GH-10423, GH-10425,
GH-10426, GH-10431

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

The arguments that broadcast are documented as float or int, but arrays
work, and the previous commit keeps them working. Document them as
float or array_like in the functions of the previous commit and in
chisquare_power, which says in its Notes that it works vectorized.

Follow-up to GH-10379, GH-10386, GH-10388, GH-10392, GH-10393, GH-10399,
GH-10405, GH-10406, GH-10411, GH-10420, GH-10421, GH-10423, GH-10425,
GH-10426, GH-10431

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

GH-10422 added the check of a negative dispersion to power_poisson_ratio_2indep
and power_negbin_ratio_2indep, and GH-10426 to power_equivalence_neginb_2indep.
power_equivalence_poisson_2indep has the same argument and had no check, so
dispersion=-1 returned nan.

Follow-up to GH-10422