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/unified_exec/process_manager.rs

@@ -160,7 +160,7 @@ impl UnifiedExecProcessManager {
         let cwd = request
             .workdir
             .clone()
-            .unwrap_or_else(|| context.turn.cwd.clone());
+            .unwrap_or_else(|| context.turn.cwd.to_path_buf());
         let process = self
             .open_session_with_sandbox(&request, cwd.clone(), context)
             .await;