eth/tracers/native: fix flatCallTracer Stop() bug

[draganm]

Fix for #29622:

Store reason when Stop is called in the flatCallTracer. This will be returned when GetResult() is called.

[s1na]

Goot catch. Pushed some changes to fix the interrupt signal. LGTM

[s1na]

Jotting down result of discussion: we can avoid all the if interrupt.Load() checks if we instead clear the function pointers on Stop. Note this would apply to all other tracers too. This is because EVM already checks for hook != nil and will not invoke those methods.