core: render remote environment cwd natively (#28152)

## Why

Model-visible `<environment_context>` should match the environment of
the executor, not of the app server.

Stacked on #28146.

## What

- Keep selected environment cwd values as `PathUri` while building
environment context.
- Render cwd text using the path convention represented by the URI, with
the canonical URI as a fallback.
- Preserve compatibility with legacy `TurnContextItem.cwd` values when
reconstructing and diffing context.
- Extend the Wine-backed remote Windows test to assert that the model
sees `powershell` and `C:\windows`.

codex-rs/exec/src/lib.rs

@@ -1423,7 +1423,11 @@ async fn parse_latest_turn_context_cwd(path: &Path) -> Option<PathBuf> {
             continue;
         };
         if let RolloutItem::TurnContext(item) = rollout_line.item {
-            return Some(item.cwd);
+            return item
+                .cwd
+                .to_abs_path()
+                .ok()
+                .map(AbsolutePathBuf::into_path_buf);
         }
     }
     None