[codex] exec-server honors remote environment cwd and shell (#28122)

## Why

Next slice needed to make progress on the `remote_env_windows` test is
to support passing a Windows cwd for the remote environment and using
that environment's native shell. This lets the test run a real Windows
process instead of only recording an early path or shell mismatch.

## What

- change `TurnEnvironmentSelection.cwd` from `AbsolutePathBuf` to
`PathUri`
- convert local cwd values to URIs when constructing selections
- preserve a remote primary cwd instead of replacing it with the local
legacy fallback
- prefer the selected environment's discovered shell for unified exec,
falling back to the session shell when unavailable
- convert back to a host-native absolute path at current native-only
consumer boundaries
- reject or deny unsupported foreign cwd values at the existing
request-permissions boundary, with TODOs for its future migration
- extend the hermetic Wine test to execute Windows PowerShell in
`C:\windows` and verify successful process completion
- record the current app-server rejection against the same Wine-backed
remote Windows fixture when its cwd is supplied as a native Windows path

codex-rs/core/src/tools/handlers/unified_exec/exec_command.rs

@@ -151,9 +151,16 @@ impl ExecCommandHandler {
         let process_id = manager.allocate_process_id().await;
         let shell_mode =
             shell_mode_for_environment(&turn.unified_exec_shell_mode, environment.as_ref());
+        // Remote environments may use a different OS and must build commands with their native
+        // shell; fall back to the session shell when the environment did not report one.
+        let shell = turn_environment
+            .shell
+            .clone()
+            .map(Arc::new)
+            .unwrap_or_else(|| session.user_shell());
         let resolved_command = get_command(
             &args,
-            session.user_shell(),
+            shell,
             &shell_mode,
             turn.config.permissions.allow_login_shell,
         )