Fix conversion from SymbolicSeries to LaurentSeries

[user202729]

Fix #39839

📝 Checklist

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⌛ Dependencies

[user202729]

A case where it raises an error with the new code

sage: x = SR.var("x")
....: f = (x^30).series(x, 20); f
Order(x^20)
sage: f.coefficients(x, sparse=True)
[[Order(x^20), 0]]

---

So another purpose of the overridden method is that it right-pad the coefficient list with zeroes on the right to match the Order. Otherwise some of the things that raise error in the new code does look like a bug.

[github-actions]

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