How do I code my arduino to return a throttle servo to idle?
If your terminal program uses key-repeat, then let each 't' operate the throttle for just slightly longer than the repeat period (slightly longer than twice the period, if you want to tolerate single dropped characters). After that period without another 't', close the throttle. One implementation would be to (re-)start a countdown timer every time you see a 't', and close the throttle when the timer goes to zero.

No auto-repeat? You'll have to simulate it with your finger (and code for a somewhat longer repeat interval, of course).