Use AbsolutePathBuf for cwd state (#15710)

Migrate `cwd` and related session/config state to `AbsolutePathBuf` so
downstream consumers consistently see absolute working directories.

Add test-only `.abs()` helpers for `Path`, `PathBuf`, and `TempDir`, and
update branch-local tests to use them instead of
`AbsolutePathBuf::try_from(...)`.

For the remaining TUI/app-server snapshot coverage that renders absolute
cwd values, keep the snapshots unchanged and skip the Windows-only cases
where the platform-specific absolute path layout differs.

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

@@ -63,6 +63,12 @@ impl AbsolutePathBuf {
         Self::from_absolute_path(current_dir)
     }
 
+    /// Construct an absolute path from `path`, resolving relative paths against
+    /// the process current working directory.
+    pub fn relative_to_current_dir<P: AsRef<Path>>(path: P) -> std::io::Result<Self> {
+        Self::resolve_path_against_base(path, std::env::current_dir()?)
+    }
+
     pub fn join<P: AsRef<Path>>(&self, path: P) -> std::io::Result<Self> {
         Self::resolve_path_against_base(path, &self.0)
     }
@@ -104,6 +110,14 @@ impl AsRef<Path> for AbsolutePathBuf {
     }
 }
 
+impl std::ops::Deref for AbsolutePathBuf {
+    type Target = Path;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
 impl From<AbsolutePathBuf> for PathBuf {
     fn from(path: AbsolutePathBuf) -> Self {
         path.into_path_buf()
@@ -216,6 +230,17 @@ mod tests {
         assert_eq!(abs_path_buf.as_path(), base_dir.join("file.txt").as_path());
     }
 
+    #[test]
+    fn relative_to_current_dir_resolves_relative_path() -> std::io::Result<()> {
+        let current_dir = std::env::current_dir()?;
+        let abs_path_buf = AbsolutePathBuf::relative_to_current_dir("file.txt")?;
+        assert_eq!(
+            abs_path_buf.as_path(),
+            current_dir.join("file.txt").as_path()
+        );
+        Ok(())
+    }
+
     #[test]
     fn guard_used_in_deserialization() {
         let temp_dir = tempdir().expect("base dir");