BUG: GroupBy.agg allows non-aggregating built-in functions

[WillAyd]

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Reproducible Example

In [38]: df = pd.DataFrame({"group": list("aab"), "val1": range(3)})

In [39]: df.groupby("group").agg("cumsum")
Out[39]: 
   val1
0     0
1     1
2     2

In [40]: df.groupby("group").agg(lambda x: x.cumsum())
ValueError: Must produce aggregated value

Issue Description

The built-in transformation functions are allowed as arguments to .agg. Haven't checked yet that this is solved by @rhshadrach work in #57706

Expected Behavior

Error for built-in functions

Installed Versions

'3.0.0.dev0+681.g434fda08cf'

[rhshadrach]

I think of this as #44845. I decided to keep it out of scope for #57706 since it could independently be done.

[WillAyd]

Ah thanks - sorry I didn't catch that. This is defintely a dupe