Reject a negative spawn_rate in /swarm instead of reporting a successful start - #3538

Problem

/swarm validated user_count for negativity but only checked spawn_rate for falsiness
(not spawn_rate), so a negative spawn_rate produced {"success": true, "message": "Swarming started"}
and then killed the spawned greenlet with ValueError: spawn_rate must be > 0, got -5.0 from
new_dispatch() — leaving runner.state == ready, runner.user_count == 0 and
unhandled_greenlet_exception == True, with the failure only visible in the log.

Fixes #3537.

Change

locust/web.py — one condition, mirroring the --spawn-rate contract (gt_zero(float) in
locust/argument_parser.py) and the adjacent user_count < 0 check:

-if user_count is None or user_count < 0 or not spawn_rate:
+if user_count is None or user_count < 0 or spawn_rate is None or spawn_rate <= 0:

spawn_rate is None or spawn_rate <= 0 rejects exactly what not spawn_rate rejected (None and
0) plus the negatives, so the existing error message and status are reused unchanged. The
shape-class branch returns before this check, so LoadShape runs that omit spawn_rate are
unaffected.

locust/test/test_web.py — test_swarm_negative_spawn_rate_returns_clean_error(), the missing
counterpart to test_swarm_negative_user_count_returns_clean_error().

How it was checked

On master at 80c03a87, Python 3.11, Windows:

• New test before the change: FAILED ... AssertionError: True is not false (i.e. /swarm replied
  success: true).
• pytest locust/test/test_web.py -k swarm -q after the change: 22 passed (includes the
  shape-class and spawn_rate=0 cases).
• pytest locust/test/test_web.py -q failure set compared against a run of the pristine tip on the
  same interpreter: identical 16 failed / 1 error, all pre-existing Windows-only ones
  (TestWebUIAuth, TestWebUIWithTLS, TestWebUIFullHistory); the only difference is
  40 passed -> 41 passed.
• ruff check locust/: all checks passed; ruff format --check: already formatted.
• mypy locust/: 5 errors, byte-identical to the tip baseline (Windows SIGWINCH / resource /
  openpty attribute errors in locust/stats.py, locust/test/__init__.py,
  locust/test/subprocess_utils.py).

Follow-up to #3527 / #3528: the dispatcher now rejects a non-positive rate, this makes the web layer
report it as an error instead of a successful start.

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