Make deliveries refused while half-open wait a cooldown (closes #306)

While the breaker was half-open, Allow refused every delivery but the
probe and CooldownRemaining returned zero, so each queued task for the
target went straight back onto the retry channel and rewrote its status
on every pass until the probe finished.

CooldownRemaining now returns the whole cooldown while half-open, so a
refused delivery waits that long. A refused delivery already at
retrying is not written again, so the retry counter now moves only
when a refusal moves a delivery into retrying.

Model: opus-5-5

internal/delivery/circuit_breaker_test.go

@@ -267,7 +267,7 @@ func TestCircuitBreaker_CooldownRemaining_ClosedReturnsZero(
 	)
 }
 
-func TestCircuitBreaker_CooldownRemaining_HalfOpenReturnsZero(
+func TestCircuitBreaker_CooldownRemaining_HalfOpenReturnsCooldown(
 	t *testing.T,
 ) {
 	t.Parallel()
@@ -282,9 +282,11 @@ func TestCircuitBreaker_CooldownRemaining_HalfOpenReturnsZero(
 
 	require.True(t, cb.Allow())
 
-	assert.Equal(t, time.Duration(0),
+	// The cooldown newShortCooldownCB gives the breaker.
+	assert.Equal(t, 50*time.Millisecond,
 		cb.CooldownRemaining(),
-		"half-open circuit should have zero cooldown remaining",
+		"a delivery refused while half-open should wait "+
+			"a whole cooldown",
 	)
 }