Large snakes are up to 10ft long but are generally non-aggressive unless hunting or provoked; they are 50% likely to be venomous.
So then... any hit will certainly count as provoking them; let us pray to Thuul they are non-venomous!
In the surprise segment
Leudigar fires his crossbow and Karl fires his horsebow:
2 # d20+3 → [22, 18], so two hits! 2 # d6 → [2, 1].
The ref rolls up some hp for the ½ HD snakes: 8 # d3 → [2, 1, 2, 3, 2, 2, 1, 1], so the first two are slain.
No reaction check is required as the snakes have now been provoked; they will attack.
No morale check is required as the snakes have not suffered one-third losses.
(DD V4 doesn't state this explicitly, but CM and the 1973 draft OD&D both state that 3" is the range at which figure can be drawn into melee. The ref takes this to mean there is no time for a missile segment in the following turn, so it's straight into melee!)
In the first melee round
the snakes have to close the distance (their charge move is 9" so distance is not an issue), but Leudigar and Karl have to switch to melee weapons, so the ref decides to dice for initiative: players, non-players: 2 # d6 → [6, 3], so the players will strike first.
Karl: d20+1 → 12, one hit; d6 → 6.
Leudigar: 2 # d20-2 → [13, 8], one hit; d6 → 6.
Well, this is a mighty feat and the remaining six snakes are butchered all in a hurry!
(Alas, the ref was hoping the Ratcatcher might Turn or beguile them, or the players might try tossing out food or fire to deter them, but none of that was necessary