Fix missing signs factor in bernoulli_logit_lpmf partials above the cutoff

[sims1253]

bernoulli_logit_lpmf computes, per observation, with signs = 2n − 1 and ntheta = signs * theta:

exp_m_ntheta = exp(-ntheta);
logp = (ntheta > cutoff)
           .select(-exp_m_ntheta,
                   (ntheta < -cutoff).select(ntheta, -log1p(exp_m_ntheta)));

edge<0>(ops_partials).partials_
    = (ntheta > cutoff)
          .select(
              -exp_m_ntheta,   // <-- bug
              (ntheta >= -cutoff)
                  .select(signs * exp_m_ntheta / (exp_m_ntheta + 1),
                          signs));

For ntheta > cutoff the value is −exp(−ntheta), so d(value)/d(ntheta) = +exp_m_ntheta, and the chain rule through ntheta = signs · theta (signs constant in theta) gives ∂lp/∂theta = signs · exp_m_ntheta. The upper partials branch returns −exp_m_ntheta — the derivative of the value without the chain-rule factor. It is correct only when signs = −1 (n = 0). For n = 1 with theta > 20 (and, symmetrically, n = 0 with theta < −20) the gradient of every such element has the wrong sign.

Eval

• Added regression test cutoff_partials_sign (test/unit/math/prim/prob/bernoulli_logit_test.cpp): for both n = 1, theta = +25 and n = 0, theta = −25 (both put ntheta above the cutoff) it checks the autodiff gradient against (a) the analytic signs * exp(−ntheta) and (b) central finite differences of the double implementation, h = 1e-3 (small enough that both FD points stay inside the same branch, large enough that the ~1e-11-magnitude values subtract cleanly).
• The test fails on unpatched code (the autodiff gradient comes back sign-flipped, off by exactly 2·exp(−ntheta)) and passes with the fix. Full test binary 6/6; the file's existing value-space cutoff test is unaffected — the bug is in partials only.

Checklist

• Copyright holder: Maximilian Scholz
  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested