test(docker): the pause between failed probes stops at the poll deadline

poll_container and the dashboard _http_probe slept a full interval after
every failed probe, even when less time than that was left, so a poll
could run up to one interval past the deadline its caller gave. Clamp
the sleep to the time remaining. The overrun never turned a failure
into a pass; it only made the deadline inexact.

Pre-existing: the poll_container sleep since c918d07b50 (#51024,
@ethernet8023, 2026-06-22); the _http_probe sleep since fb51253620
(#34188, @benbarclay, 2026-05-29).

tests/docker/conftest.py

@@ -257,7 +257,7 @@ def poll_container(
         last = r.stdout
         if r.returncode == 0:
             return True, last
-        time.sleep(interval_s)
+        time.sleep(min(interval_s, max(0.0, end - time.monotonic())))
     if not completed:
         raise AssertionError(f"No probe of {container} completed within {deadline_s}s; {last}")
     return False, last

tests/docker/test_dashboard.py

@@ -113,7 +113,7 @@ except urllib.error.HTTPError as h:
                 last_err = f"parse: {exc!r} / stdout={r.stdout!r}"
         else:
             last_err = f"rc={r.returncode} stderr={r.stderr!r}"
-        time.sleep(0.5)
+        time.sleep(min(0.5, max(0.0, end - time.monotonic())))
     raise AssertionError(
         f"Probe of {path} never returned HTTP within {deadline_s}s; "
         f"last error: {last_err}"

tests/test_docker_poll_stalled_exec.py

@@ -104,3 +104,18 @@ def test_stall_after_a_completed_probe_keeps_its_output(monkeypatch):
     failed = SimpleNamespace(returncode=1, stdout="HOME=/root", stderr="")
     monkeypatch.setattr(docker_conftest, "docker_exec_sh", _scripted_exec(clock, [failed, "stall"]))
     assert docker_conftest.poll_container("c", "probe", deadline_s=12.0) == (False, "HOME=/root")
+
+
+@pytest.mark.parametrize("module", [docker_conftest, test_dashboard], ids=["poll_container", "_http_probe"])
+def test_sleep_between_failed_probes_stops_at_the_deadline(monkeypatch, module):
+    """The pause after a failed probe never runs past the deadline the caller gave."""
+    clock = _Clock()
+    monkeypatch.setattr(module, "time", clock)
+    failed = SimpleNamespace(returncode=1, stdout="", stderr="")
+    monkeypatch.setattr(module, "docker_exec_sh", _scripted_exec(clock, [failed]))
+    if module is docker_conftest:
+        assert module.poll_container("c", "probe", deadline_s=1.0) == (False, "")
+    else:
+        with pytest.raises(AssertionError, match="never returned HTTP"):
+            module._http_probe("c", "/api/status", deadline_s=1.0)
+    assert clock.now <= 1.0