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/tui/src/test_support.rs

@@ -0,0 +1,28 @@
+use codex_utils_absolute_path::AbsolutePathBuf;
+use std::path::Path;
+use std::path::PathBuf;
+
+pub(crate) trait PathExt {
+    fn abs(&self) -> AbsolutePathBuf;
+}
+
+impl PathExt for Path {
+    fn abs(&self) -> AbsolutePathBuf {
+        AbsolutePathBuf::try_from(self.to_path_buf())
+            .unwrap_or_else(|_| panic!("path should already be absolute"))
+    }
+}
+
+pub(crate) trait PathBufExt {
+    fn abs(&self) -> AbsolutePathBuf;
+}
+
+impl PathBufExt for PathBuf {
+    fn abs(&self) -> AbsolutePathBuf {
+        self.as_path().abs()
+    }
+}
+
+pub(crate) fn test_path_display(path: &str) -> String {
+    Path::new(path).abs().display().to_string()
+}