exponential_mt / exponential_rt with negative rate

Should allow exponential_mt(mean, ub) to have mean >= 0.5 * ub.
Likewise should allow exponential_rt(theta, ub) to have theta <= 0.

Locate the definitions and parameter validation for exponential_mt(mean, ub) and exponential_rt(theta, ub). Check how the current bounds are enforced, then update the behavior so the requested boundary values are accepted and verify both functions across the stated limits.

Written by the indexing model from the issue text.