`test_hedged_requests/test.py::test_stuck_replica` is chronically flaky
on heavily overcommitted ASan/UBSan integration shards. The failure shape
in CIDB is identical across 9 unrelated PRs in the past 30 days:

    pause_container('node_1') did not become observably effective within
    90.0s ... still succeed after ~890 probe iterations; last outcome:
    server replied (4 bytes)

The probe in `_wait_for_pause_effective` keeps receiving live ClickHouse
exception packets from `node_1` for the entire 90s budget, which proves
that `docker compose pause` returned success while the cgroup freezer
never actually froze the user-space tasks. The existing SIGSTOP fallback
in `ClickHouseCluster.pause_container` only fires when `docker compose
pause` raises an exception, not when it lies about success, so the
escalation path was never taken.

Switch this test to `cluster.pause_container_using_signal`, which uses
`SIGSTOP`/`SIGCONT` from the start. A signal cannot be silently lost the
way the cgroup freezer can, so the pause becomes observable on the very
first probe iteration. Same helper is already used by
`test_postgresql_replica_database_engine` for the same reason.

The change is local to one call site. The chronic flake hit
`Integration tests (amd_asan_ubsan, db disk, old analyzer, 1/6)` in the
following PRs (CIDB, last 30 days): ClickHouse#103404, ClickHouse#106311, ClickHouse#105010, ClickHouse#105056,
ClickHouse#104830, ClickHouse#106025, ClickHouse#100391, ClickHouse#106571, ClickHouse#103229. All show the same
`server replied (4 bytes)` outcome, distinguishing this from generic
flakiness.

Related: ClickHouse#106571