fix: the lane sizer reads the live pool, so it stops growing a lane past its cap (4409)

It read celery's `max-concurrency` as the pool size, but prefork reports that
as the limit the pool booted with. pool_grow and pool_shrink never update it.
A lane booted at 1 therefore read 1 forever. The sweep sent `target - 1` on
every tick while work waited, and could never shrink, since 1 - 1 is 0. The
System tab showed Scheduler "6 / 1" at a cap of 2.

pool_size() counts `processes` and falls back to the limit only when there is
no process list.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_worker_control.py

@@ -671,3 +671,36 @@ def test_an_unknown_worker_is_still_ignored_rather_than_guessed_at(monkeypatch):
     live = wc.inspect_lanes_sync()
 
     assert all(not s.present for s in live.values())
+
+
+# --- the pool size is the processes running, not the boot limit ---------------
+#
+# The operator's System tab, 2026-09-24: Scheduler "6 / 1" at a cap of 2. Celery
+# reports `max-concurrency` as the limit the pool BOOTED with; pool_grow and
+# pool_shrink never update it. The sweep read 1 forever, sent `target - 1` on
+# every tick, and grew the lane past its cap with no way to shrink it back.
+
+
+def test_pool_size_counts_the_live_processes_not_the_boot_limit():
+    """After two pool_grow calls on a pool booted at 1."""
+    assert wc.pool_size({"max-concurrency": 1, "processes": [11, 12, 13]}) == 3
+
+
+def test_pool_size_falls_back_to_the_limit_without_a_process_list():
+    assert wc.pool_size({"max-concurrency": 4}) == 4
+    assert wc.pool_size({}) is None
+
+
+def test_a_grown_pool_reads_at_its_real_size(monkeypatch):
+    """The read the sweep sizes from: a pool at its target must read AT its
+    target, or `target - current` never reaches zero and every tick grows it."""
+    _stub_active_queues(monkeypatch, {"scheduler@h": []})
+    import sys
+    insp = sys.modules["backend.app.celery_app"].celery.control.inspect
+    insp.stats = lambda self: {
+        "scheduler@h": {"pool": {"max-concurrency": 1, "processes": [1, 2]}},
+    }
+
+    live = wc.inspect_lanes_sync()
+
+    assert live["scheduler"].pool == 2