Sometimes conversion from Laurent series to rational function field raise error

[user202729]

This is not perfect yet. Ideally it should change_ring the Laurent series to the target field before doing the approximation.

R.<x> = QQ[]
T.<t> = R[[]]
f = 1 + (x + 1)*t + (x^2 + 2*x + 1)*t^2 + (x^3 + 3*x^2 + 3*x + 1)*t^3 + O(t^4)  # belong to R[[t]], not Frac(R)[[t]]
Frac(Frac(R)[t])( f )  # error!
Frac(Frac(R)[t])( Frac(R)[[t]]( f ) )  # work

We want the # error! line to work just like the last line.

Originally posted by @user202729 in #39485 (comment)