Revert pyproject.toml modifications on Ctrl-C (#7024)

## Summary

Not perfect, but an improvement at least for an interactive experience.

Closes #6818.

crates/uv/Cargo.toml

@@ -48,6 +48,7 @@ anstream = { workspace = true }
 anyhow = { workspace = true }
 axoupdater = { workspace = true, features = ["github_releases", "tokio"], optional = true }
 clap = { workspace = true, features = ["derive", "string", "wrap_help"] }
+ctrlc = { workspace = true }
 flate2 = { workspace = true, default-features = false }
 fs-err = { workspace = true, features = ["tokio"] }
 futures = { workspace = true }

crates/uv/src/commands/project/add.rs

@@ -488,6 +488,25 @@ pub(crate) async fn add(
         .with_pyproject_toml(toml::from_str(&content).map_err(ProjectError::TomlParse)?)
         .ok_or(ProjectError::TomlUpdate)?;
 
+    // Set the Ctrl-C handler to revert changes on exit.
+    let _ = ctrlc::set_handler({
+        let root = root.clone();
+        let existing = existing.clone();
+        move || {
+            // Revert the changes to the `pyproject.toml`, if necessary.
+            if modified {
+                let _ = fs_err::write(root.join("pyproject.toml"), &existing);
+            }
+
+            #[allow(clippy::exit, clippy::cast_possible_wrap)]
+            std::process::exit(if cfg!(windows) {
+                0xC000_013A_u32 as i32
+            } else {
+                130
+            });
+        }
+    });
+
     match lock_and_sync(
         project,
         &mut toml,
@@ -518,15 +537,15 @@ pub(crate) async fn add(
 
             // Revert the changes to the `pyproject.toml`, if necessary.
             if modified {
-                fs_err::write(root.join("pyproject.toml"), existing)?;
+                fs_err::write(root.join("pyproject.toml"), &existing)?;
             }
 
             Ok(ExitStatus::Failure)
         }
         Err(err) => {
             // Revert the changes to the `pyproject.toml`, if necessary.
             if modified {
-                fs_err::write(root.join("pyproject.toml"), existing)?;
+                fs_err::write(root.join("pyproject.toml"), &existing)?;
             }
             Err(err.into())
         }