[codex] core: restore absolute turn context cwd (#28629)

## Why

#28152 jumped the gun on moving the rollout format to store URIs, and
would likely break compat with some features that don't go through the
same types as the core logic.

## What

Make `TurnContextItem.cwd` an `AbsolutePathBuf` again, remove test added
for `PathUri` serialization in rollouts. Also drops a bunch of error
paths that are no longer needed.

codex-rs/core/src/codex_thread.rs

@@ -458,7 +458,7 @@ impl CodexThread {
             self.codex
                 .session
                 .record_context_updates_and_set_reference_context_item(turn_context.as_ref())
-                .await?;
+                .await;
         }
         self.codex
             .session