No overload variant of "__add__" of "Timestamp" matches argument type "Series[Timedelta]"

[randolf-scholz]

import pandas as pd

s = pd.Series(pd.to_timedelta([1, 2, 3], "h"))
pd.Timestamp(0) + s.cumsum()

with mypy

$ mypy tmp.py 
tmp.py:4:1: error: No overload variant of "__add__" of "Timestamp" matches argument type "Series[Timedelta]"  [operator]
tmp.py:4:1: note: Possible overload variants:
tmp.py:4:1: note:     def __add__(self, ndarray[tuple[int, ...], dtype[timedelta64[timedelta | int | None]]], /) -> ndarray[tuple[int, ...], dtype[datetime64[date | int | None]]]
tmp.py:4:1: note:     def __add__(self, timedelta | timedelta64[timedelta | int | None] | Tick, /) -> Timestamp
tmp.py:4:1: note:     def __add__(self, TimedeltaSeries, /) -> TimestampSeries
tmp.py:4:1: note:     def __add__(self, TimedeltaIndex, /) -> DatetimeIndex
Found 1 error in 1 file (checked 1 source file)

with pyright

$ pyright tmp.py 
/home/rscholz/github/randolf-scholz/tsdm/tmp.py
  /home/rscholz/github/randolf-scholz/tsdm/tmp.py:4:1 - error: Operator "+" not supported for types "Timestamp" and "Series[Timedelta]" (reportOperatorIssue)
  /home/rscholz/github/randolf-scholz/tsdm/tmp.py:4:1 - warning: Expression value is unused (reportUnusedExpression)
1 error, 1 warning, 0 informations 

mypy                          1.16.0
mypy_extensions               1.1.0
pandas                        2.3.0
pandas-stubs                  2.2.3.250527
pyright                       1.1.401

[Dr-Irv]

We have the correct overload for TimedeltaSeries.__radd__(), but cumsum() is saying it is a Series[Timedelta], so we need to add a cumsum() method to TimedeltaSeries() that returns a TimedeltaSeries.

PR welcome.