Reject a non-positive rate in constant_throughput(), using the same guard poisson() in
this module already has.

Without it:

• constant_throughput(-0.1) becomes constant_pacing(-10), whose max(0, wait_time - run_time)
  returns 0 every time, so the load runs flat out with no warning — the opposite of what was
  configured;
• constant_throughput(0) raises a bare ZeroDivisionError: division by zero while the locustfile
  is imported, naming neither the function nor the argument.

constant_throughput() is documented as the mathematical inverse of constant_pacing() and the
fixed-rate sibling of poisson(), so the positive-rate requirement belongs in the same place.

Fixes #3533

How was it checked

On master at d7842f50, before the change (baseline file restored from HEAD):

locust/test/test_wait_time.py  ->  1 failed, 14 passed
FAILED TestWaitTime::test_constant_throughput_invalid_rate
    return constant_pacing(1 / task_runs_per_second)
E   ZeroDivisionError: division by zero

after the change:

locust/test/test_wait_time.py  ->  15 passed
ruff check locust/             ->  All checks passed
ruff format --check            ->  2 files already formatted
mypy locust/user/wait_time.py  ->  Success: no issues found in 1 source file

No other call site in the repository passes a non-positive rate, and the documented examples
(constant_throughput(0.1), constant_throughput(2), constant_throughput(10)) are unaffected.

Drafted with AI assistance in this session; no human reviewed it before submission.