test: sound backoff-jitter assertions (per-delay band + non-overlapping growth) and scoped import-stub teardown in the pairing regression tests (F5 residual)

tests/test_connection.py

@@ -211,15 +211,22 @@ class TestReconnectBackoff:
         manager = _manager(poller)
 
         delays = []
+        backoffs = []
         backoff = 5.0
         for _ in range(7):
+            backoffs.append(backoff)
             delays.append(await manager._watchdog_cycle(backoff))
             backoff = min(backoff * 2, 300.0)
 
-        # 5→10→20→40→80→160→300 growth with easing; the last must not exceed
-        # the cap (with jitter).
-        assert max(delays) <= 300.0 * 1.25
-        assert delays[1] >= delays[0] * 1.5
+        # F5 soundness: the previous ±25 % jitter could flake the claim
+        # delays[1] > delays[0] * 1.5: BACKOFF=10 jittered to 9.17 vs
+        # BACKOFF=5 jittered to 6.25 → 6.25*1.5 = 9.375). Assert only what
+        # jitter guarantees: each delay sits inside ITS OWN backoff's band
+        # (non-overlapping consecutive bands ⇒ delays[1] >= delays[0],
+        # since min next = 7.5 >= max first = 6.25), and never above the cap.
+        for backoff, delay in zip(backoffs, delays, strict=True):
+            assert 0.75 * backoff <= delay <= 1.25 * backoff
+        assert delays[1] >= delays[0]
         assert delays[-1] <= 300.0 * 1.25
 
     async def test_repeated_failures_do_not_spam_listeners(self):