`uv add` should roll back changes to `pyproject.toml` if the automatic sync is cancelled

[akx]

uv 0.4.0, macOS.

A minimal code snippet that reproduces the issue:

$ uv init foo
$ cd foo
$ uv add pyobjc
💭 oh crud, I don't need every single pyobjc framework! that's a lot of framework!
^C
$ uv add pyobjc-core
💭 oh crud, it's still downloading all frameworks! what's happening?!
^C
$ cat pyproject.toml
[...]
dependencies = [
    "pyobjc",
    "pyobjc-core",
]
$

IOW, I'd expect that if I cancel the uv add operation (even if it's made up of add and implicit sync afterwards), pyproject.toml was back to the state it was before uv add.

[charliermarsh]

(We correctly do this if lock or sync fail, but not if the program is interrupted.)