The Dire Rat take a 5' step towards the Bugbear to avoid an AOO and bites at the Bugbear.
Bite vs. AC 17 1d20 + 3 ⇒ (16) + 3 = 19 Damage 1d4 + 2 ⇒ (4) + 2 = 6 Disease DC 11 1d20 + 6 ⇒ (18) + 6 = 24
The Dire Rat bites down hard on the leg of the Bugbear! The Bugbear does avoid a nasty disease though.
SQUEAK!
And with that the Summoned Augmented Celestial Dire Rat disappears back to the plane of existence it was summoned from at the end of the round.