[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/thread_manager_tests.rs

@@ -23,6 +23,7 @@ use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::ThreadSource;
 use codex_protocol::protocol::TurnStartedEvent;
 use codex_protocol::protocol::UserMessageEvent;
+use codex_utils_path_uri::PathUri;
 use core_test_support::PathBufExt;
 use core_test_support::PathExt;
 use core_test_support::responses::mount_models_once;
@@ -331,7 +332,7 @@ async fn start_thread_rejects_explicit_local_environment_when_default_provider_i
             parent_trace: None,
             environments: vec![TurnEnvironmentSelection {
                 environment_id: "local".to_string(),
-                cwd: config.cwd.clone(),
+                cwd: PathUri::from_abs_path(&config.cwd),
             }],
             thread_extension_init: Default::default(),
         })
@@ -594,7 +595,7 @@ async fn resume_and_fork_do_not_restore_thread_environments_from_rollout() {
     std::fs::create_dir_all(&selected_cwd).expect("create selected cwd");
     let environments = vec![TurnEnvironmentSelection {
         environment_id: "local".to_string(),
-        cwd: selected_cwd.clone(),
+        cwd: PathUri::from_abs_path(&selected_cwd),
     }];
     let default_cwd = config.cwd.clone();
     let mut source_config = config.clone();