BUG: Fix new DTypes and new string promotion when signature is involved #26887
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Backport of #26744.
Unfortunately, did a bit of cleanup to make this work, so this needs a bit of a careful look.
I.e. the first commit moves the "allow legacy promotion" logic to later, where the signature is more readily available. That shouldn't change anything else (the only reason it is used earlier is to decide if checking for "should use scalar" makes sense, but that code handles unknown dtypes just fine).
It then fixes the string add promoter to close the issue.
Closes #26735