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/hook_runtime.rs

@@ -92,7 +92,7 @@ pub(crate) async fn run_pending_session_start_hooks(
 
     let request = codex_hooks::SessionStartRequest {
         session_id: sess.conversation_id,
-        cwd: turn_context.cwd.clone(),
+        cwd: turn_context.cwd.to_path_buf(),
         transcript_path: sess.hook_transcript_path().await,
         model: turn_context.model_info.slug.clone(),
         permission_mode: hook_permission_mode(turn_context),
@@ -120,7 +120,7 @@ pub(crate) async fn run_pre_tool_use_hooks(
     let request = PreToolUseRequest {
         session_id: sess.conversation_id,
         turn_id: turn_context.sub_id.clone(),
-        cwd: turn_context.cwd.clone(),
+        cwd: turn_context.cwd.to_path_buf(),
         transcript_path: sess.hook_transcript_path().await,
         model: turn_context.model_info.slug.clone(),
         permission_mode: hook_permission_mode(turn_context),
@@ -149,7 +149,7 @@ pub(crate) async fn run_user_prompt_submit_hooks(
     let request = UserPromptSubmitRequest {
         session_id: sess.conversation_id,
         turn_id: turn_context.sub_id.clone(),
-        cwd: turn_context.cwd.clone(),
+        cwd: turn_context.cwd.to_path_buf(),
         transcript_path: sess.hook_transcript_path().await,
         model: turn_context.model_info.slug.clone(),
         permission_mode: hook_permission_mode(turn_context),