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/hierarchical_agents.rs

@@ -23,7 +23,14 @@ async fn hierarchical_agents_appends_to_project_doc_in_user_instructions() {
             .features
             .enable(Feature::ChildAgentsMd)
             .expect("test config should allow feature update");
-        std::fs::write(config.cwd.join("AGENTS.md"), "be nice").expect("write AGENTS.md");
+        std::fs::write(
+            config
+                .cwd
+                .join("AGENTS.md")
+                .expect("absolute AGENTS.md path"),
+            "be nice",
+        )
+        .expect("write AGENTS.md");
     });
     let test = builder.build(&server).await.expect("build test codex");