Why does fft energy leakage was solved
See my answer here on how to work with the units in question:
DFT exercise in the book Understanding digital signal processing 3 Ed
Then apply the fact that leakage does not occur when the DFT frame covers a whole number of cycles of your tone signal.

$$ Fs = 1000000 \frac{Samples}{Second}  $$
$$ Fd = 113280  \frac{Cycles}{Second} $$
$$ N = 256 \frac{Samples}{Frame} $$
$$ \frac{Fd}{Fs} \cdot N = \frac{\frac{Cycles}{Second}}{\frac{Samples}{Second}} \cdot \frac{Samples}{Frame} = \frac{Cycles}{Frame}  $$
$$ \frac{113280}{1000000} \cdot 256 =  28.99968 $$
Which is pretty darn close to a whole number.