encoding/json: Unmarshal leaves ±Inf in the destination on float overflow in Go 1.27 [1.27 backport]

[gopherbot]

@dsnet requested issue #81062 to be considered for backport to the next 1.27 minor release.

@gopherbot Please backport to 1.27. This is a regression in encoding/json causing it to store Inf when a parsing a value that overflows.

[dsnet]

#81062 contains two stacked bugs:

1. The backing float value is mutated even with an error parsing a float.
2. Which backing float value is used is inconsistent with how Go 1.26 behaved.

Bug 1 already has a fixed merged in CL/820580. I'm confident in the correctness of this fix and we should cherrypick it.

Bug 2 is really obscure and I'm not confident in the proposed fixes for them. That regression may need to wait until Go 1.28 to fix OR we just say the new behavior is here to stay.

[IBlackVoid]

This got swept to 1.27.2 on the 1st, and it only needs bug 1.

@dsnet said above he is confident in CL 820580 and that we should cherrypick it. That is merged on master as 94b71ff. The doubt in the rest of his comment is about bug 2, which is a separate fix.

Bug 2 is already out of 1.27, it is on the Go1.28 milestone in #81062. Nothing here waits on it.

I did not write 820580 so I cannot make the cherrypick myself.