Revert changes to pyproject.toml when sync fails duing uv add (#6526)

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
This is a attempt at fixing https://github.com/astral-sh/uv/issues/6486.
It reverts changes made to `pyproject.toml` when sync fails during `uv
add`. This solution felt a little heavy handed and could probably be
improved but it is what happens when locking fails during `uv add` so I
thought it would be a good start.

## Test Plan

<!-- How was it tested? -->

I have added a test case for this to `tests/edit.rs`. It uses
`pytorch==1.0.2` to achieve the desired failure.

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

@@ -597,7 +597,7 @@ pub(crate) async fn add(
     // Initialize any shared state.
     let state = SharedState::default();
 
-    project::sync::do_sync(
+    if let Err(err) = project::sync::do_sync(
         &project,
         &venv,
         &lock,
@@ -613,7 +613,14 @@ pub(crate) async fn add(
         cache,
         printer,
     )
-    .await?;
+    .await
+    {
+        // Revert the changes to the `pyproject.toml`, if necessary.
+        if modified {
+            fs_err::write(project.root().join("pyproject.toml"), existing)?;
+        }
+        return Err(err.into());
+    }
 
     Ok(ExitStatus::Success)
 }

crates/uv/tests/edit.rs

@@ -3873,3 +3873,69 @@ fn add_git_to_script() -> Result<()> {
     });
     Ok(())
 }
+
+// Revert changes to pyproject.toml if add fails
+#[test]
+fn fail_to_add_revert_project() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+    "#})?;
+
+    // Adding `pytorch==1.0.2` should produce an error
+    let filters = std::iter::once((r"exit code: 1", "exit status: 1"))
+        .chain(context.filters())
+        .collect::<Vec<_>>();
+    uv_snapshot!(filters, context.add(&["pytorch==1.0.2"]), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    error: Failed to prepare distributions
+      Caused by: Failed to fetch wheel: pytorch==1.0.2
+      Caused by: Build backend failed to build wheel through `build_wheel()` with exit status: 1
+    --- stdout:
+
+    --- stderr:
+    Traceback (most recent call last):
+      File "<string>", line 11, in <module>
+      File "[CACHE_DIR]/builds-v0/[TMP]/build_meta.py", line 410, in build_wheel
+        return self._build_with_temp_dir(
+               ^^^^^^^^^^^^^^^^^^^^^^^^^^
+      File "[CACHE_DIR]/builds-v0/[TMP]/build_meta.py", line 395, in _build_with_temp_dir
+        self.run_setup()
+      File "[CACHE_DIR]/builds-v0/[TMP]/build_meta.py", line 487, in run_setup
+        super().run_setup(setup_script=setup_script)
+      File "[CACHE_DIR]/builds-v0/[TMP]/build_meta.py", line 311, in run_setup
+        exec(code, locals())
+      File "<string>", line 15, in <module>
+    Exception: You tried to install "pytorch". The package named for PyTorch is "torch"
+    ---
+    "###);
+
+    let pyproject_toml = fs_err::read_to_string(context.temp_dir.join("pyproject.toml"))?;
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+        "###
+        );
+    });
+
+    Ok(())
+}