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/suite/resume_warning.rs

@@ -28,7 +28,7 @@ fn resume_history(
     let turn_ctx = TurnContextItem {
         turn_id: Some(turn_id.clone()),
         trace_id: None,
-        cwd: config.cwd.clone(),
+        cwd: config.cwd.to_path_buf(),
         current_date: None,
         timezone: None,
         approval_policy: config.permissions.approval_policy.value(),