gh-151845: Fix formatfloat() return-value contract in unicode_format.c

[aisk]

As mentioned in the issue, this don't affected to callers and users, so /skip-news and don't requires a test.

• Issue: formatfloat wrong return in unicode_format.c #151845

[sobolevn]

Does it need to be backported?

[aisk]

This change doesn’t affect Python’s behavior because the caller already handles it. However, I think it would still be better to backport it to avoid potential future conflicts, as long as the backport is conflict-free.

[miss-islington-app]

Thanks @aisk for the PR, and @sobolevn for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @aisk and @sobolevn, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c2661e6189ab2833bf32fa723cc427e64c026839 3.14

[bedevere-app]

GH-151865 is a backport of this pull request to the 3.15 branch.

[miss-islington-app]

Sorry, @aisk and @sobolevn, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c2661e6189ab2833bf32fa723cc427e64c026839 3.13

[aisk]

Hi, since backporting this to 3.13 and 3.14 causes conflicts, I think we should just leave it as is.

[StanFromIreland]

@aisk feel free to remove the labels if a backport is unsuitable/not worth it.

[aisk]

@aisk feel free to remove the labels if a backport is unsuitable/not worth it.

TIL, thanks!