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/onboarding/onboarding_screen.rs

@@ -82,7 +82,7 @@ impl OnboardingScreen {
             auth_manager,
             config,
         } = args;
-        let cwd = config.cwd.clone();
+        let cwd = config.cwd.to_path_buf();
         let forced_chatgpt_workspace_id = config.forced_chatgpt_workspace_id.clone();
         let forced_login_method = config.forced_login_method;
         let codex_home = config.codex_home.clone();