I just defeated the Yaun-Ti in Chapter one.
Anyway, I found a room full of chests, one of which has a fire trap on it. If I try to bash it, it explodes and I get killed. My character is a Neutral Good Fighter and has to lock-pick/disable-trap capabilities. Is there a henchmen that can do it for me? If so, who is he/she and where can he/she be found?

To pick the lock get Tomi Undergallows in the Hall of Justice. He is righjt before the main door as you leave.

If you have a ranged weapon you can stand back and shoot at it also.

I have been playing the whole game so far with a lone fighter I'm now in chapter 3 and have bashed open every chest and door that are locked or trapped. The trap damage is random so if you just save and keep on trying to open it you should eventually survive unless you have a very low constitution mine is only 12.

normally if a chest got super high trap damage, it doesn't have anything good in it. ;)

some chests can't be bashed. and that NPC rogue is no match for a PC rogue. That's the best way to open them ;)

My first character (who I beat the game with) was a Fighter who multi-classed as rogue

He could wear full armour, use all martial weapons and did lots of sneak attack damage if he flanked someone :D

not to mention trap disarming and lockpicking