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/src/tools/runtimes/shell/unix_escalation.rs

@@ -159,7 +159,7 @@ pub(super) async fn try_run_zsh_fork(
         network: sandbox_network,
         windows_sandbox_level,
         arg0,
-        sandbox_policy_cwd: ctx.turn.cwd.clone(),
+        sandbox_policy_cwd: ctx.turn.cwd.to_path_buf(),
         macos_seatbelt_profile_extensions: ctx
             .turn
             .config
@@ -263,7 +263,7 @@ pub(crate) async fn prepare_unified_exec_zsh_fork(
         network: exec_request.network.clone(),
         windows_sandbox_level: exec_request.windows_sandbox_level,
         arg0: exec_request.arg0.clone(),
-        sandbox_policy_cwd: ctx.turn.cwd.clone(),
+        sandbox_policy_cwd: ctx.turn.cwd.to_path_buf(),
         macos_seatbelt_profile_extensions: ctx
             .turn
             .config