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/codex_tests_guardian.rs

@@ -23,7 +23,8 @@ use codex_protocol::models::ResponseItem;
 use codex_protocol::models::function_call_output_content_items_to_text;
 use codex_protocol::permissions::FileSystemSandboxPolicy;
 use codex_protocol::permissions::NetworkSandboxPolicy;
-use codex_utils_absolute_path::AbsolutePathBuf;
+use core_test_support::PathExt;
+use core_test_support::TempDirExt;
 use core_test_support::codex_linux_sandbox_exe_or_skip;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
@@ -123,7 +124,7 @@ async fn guardian_allows_shell_additional_permissions_requests_past_policy_valid
                 "echo hi".to_string(),
             ]
         },
-        cwd: turn_context.cwd.clone(),
+        cwd: turn_context.cwd.to_path_buf(),
         expiration: expiration_ms.into(),
         capture_policy: ExecCapturePolicy::ShellTool,
         env: HashMap::new(),
@@ -388,12 +389,11 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
     .expect("write policy file");
 
     let mut config = build_test_config(codex_home.path()).await;
-    config.cwd = project_dir.path().to_path_buf();
+    config.cwd = project_dir.abs();
     config.config_layer_stack = ConfigLayerStack::new(
         vec![ConfigLayerEntry::new(
             ConfigLayerSource::Project {
-                dot_codex_folder: AbsolutePathBuf::from_absolute_path(project_dir.path())
-                    .expect("absolute project path"),
+                dot_codex_folder: project_dir.path().abs(),
             },
             toml::Value::Table(Default::default()),
         )],