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/status/helpers.rs

@@ -46,7 +46,7 @@ pub(crate) fn compose_agents_summary(config: &Config) -> String {
                     .map(|name| name.to_string_lossy().to_string())
                     .unwrap_or_else(|| "<unknown>".to_string());
                 let display = if let Some(parent) = p.parent() {
-                    if parent == config.cwd {
+                    if parent == config.cwd.as_path() {
                         file_name.clone()
                     } else {
                         let mut cur = config.cwd.as_path();