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/core/src/session/turn_context.rs

@@ -396,10 +396,17 @@ impl TurnContext {
 
     pub(crate) fn to_turn_context_item(&self) -> TurnContextItem {
         let workspace_roots = self.config.effective_workspace_roots();
+        let cwd = self
+            .environments
+            .primary()
+            .map(|environment| environment.cwd().clone())
+            .unwrap_or_else(|| {
+                #[allow(deprecated)]
+                PathUri::from_abs_path(&self.cwd)
+            });
         TurnContextItem {
             turn_id: Some(self.sub_id.clone()),
-            #[allow(deprecated)]
-            cwd: self.cwd.to_path_buf(),
+            cwd,
             workspace_roots: (!workspace_roots.is_empty()).then_some(workspace_roots),
             current_date: self.current_date.clone(),
             timezone: self.timezone.clone(),