app-server: preserve target-native environment cwd (#28146)

## Why

app-server may run on a different OS from the selected exec-server
environment. Parsing that environment’s cwd with the Codex host’s path
rules prevents thread startup.

## What

Carry environment cwd values as `LegacyAppPathString` at the app-server
boundary and `PathUri` internally. Existing tool-call schemas and
relative-path behavior stay host-native; remaining local-only consumers
convert explicitly and leave follow-up TODOs.

The Wine integration test verifies app-server can start a thread and
complete an ordinary turn with a Windows environment cwd from Linux.

## Validation

- `bazel test //codex-rs/core/tests/remote_env_windows:smoke-test
--test_output=errors`
- focused app-server environment-selection and protocol schema tests
- scoped Clippy for `codex-core` and `codex-app-server-protocol`

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -3723,7 +3723,14 @@ fn thread_settings_update_params_preserve_field_level_experimental_gates() {
 
 #[test]
 fn turn_start_params_round_trip_environments() {
-    let cwd = test_absolute_path();
+    // Use a path foreign to the test host so this exercises syntax preservation instead of the
+    // host-native conversion performed by test_absolute_path().
+    #[cfg(windows)]
+    let raw_cwd = "/workspace";
+    #[cfg(not(windows))]
+    let raw_cwd = r"C:\workspace";
+    let cwd: LegacyAppPathString =
+        serde_json::from_value(json!(raw_cwd)).expect("API path should deserialize");
     let params: TurnStartParams = serde_json::from_value(json!({
         "threadId": "thread_123",
         "input": [],
@@ -3805,27 +3812,6 @@ fn turn_start_params_treat_null_or_omitted_environments_as_default() {
     );
 }
 
-#[test]
-fn turn_start_params_reject_relative_environment_cwd() {
-    let err = serde_json::from_value::<TurnStartParams>(json!({
-        "threadId": "thread_123",
-        "input": [],
-        "environments": [
-            {
-                "environmentId": "local",
-                "cwd": "relative"
-            }
-        ],
-    }))
-    .expect_err("relative environment cwd should fail");
-
-    assert!(
-        err.to_string()
-            .contains("AbsolutePathBuf deserialized without a base path"),
-        "unexpected error: {err}"
-    );
-}
-
 #[test]
 fn realtime_append_text_defaults_role_to_user() {
     let params = serde_json::from_value::<ThreadRealtimeAppendTextParams>(json!({