chore: enusre the logic that creates ConfigLayerStack has access to cwd (#8353)

`load_config_layers_state()` should load config from a
`.codex/config.toml` in any folder between the `cwd` for a thread and
the project root. Though in order to do that,
`load_config_layers_state()` needs to know what the `cwd` is, so this PR
does the work to thread the `cwd` through for existing callsites.

A notable exception is the `/config` endpoint in app server for which a
`cwd` is not guaranteed to be associated with the query, so the `cwd`
param is `Option<AbsolutePathBuf>` to account for this case.

The logic to make use of the `cwd` will be done in a follow-up PR.

codex-rs/utils/absolute-path/src/lib.rs

@@ -34,6 +34,11 @@ impl AbsolutePathBuf {
         Ok(Self(absolute_path.into_owned()))
     }
 
+    pub fn current_dir() -> std::io::Result<Self> {
+        let current_dir = std::env::current_dir()?;
+        Self::from_absolute_path(current_dir)
+    }
+
     pub fn join<P: AsRef<Path>>(&self, path: P) -> std::io::Result<Self> {
         Self::resolve_path_against_base(path, &self.0)
     }