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/core/tests/common/lib.rs

@@ -14,6 +14,7 @@ use codex_core::config::ConfigBuilder;
 use codex_core::config::ConfigOverrides;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use regex_lite::Regex;
+use std::path::Path;
 use std::path::PathBuf;
 
 pub mod apps_test_server;
@@ -104,6 +105,36 @@ pub fn test_absolute_path(unix_path: &str) -> AbsolutePathBuf {
     test_absolute_path_with_windows(unix_path, /*windows_path*/ None)
 }
 
+pub trait PathExt {
+    fn abs(&self) -> AbsolutePathBuf;
+}
+
+impl PathExt for Path {
+    fn abs(&self) -> AbsolutePathBuf {
+        AbsolutePathBuf::try_from(self.to_path_buf()).expect("path should already be absolute")
+    }
+}
+
+pub trait PathBufExt {
+    fn abs(&self) -> AbsolutePathBuf;
+}
+
+impl PathBufExt for PathBuf {
+    fn abs(&self) -> AbsolutePathBuf {
+        self.as_path().abs()
+    }
+}
+
+pub trait TempDirExt {
+    fn abs(&self) -> AbsolutePathBuf;
+}
+
+impl TempDirExt for TempDir {
+    fn abs(&self) -> AbsolutePathBuf {
+        self.path().abs()
+    }
+}
+
 pub fn test_tmp_path() -> AbsolutePathBuf {
     test_absolute_path_with_windows("/tmp", Some(r"C:\Users\codex\AppData\Local\Temp"))
 }