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/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -17325,7 +17325,7 @@
     "TurnEnvironmentParams": {
       "properties": {
         "cwd": {
-          "$ref": "#/definitions/AbsolutePathBuf"
+          "$ref": "#/definitions/LegacyAppPathString"
         },
         "environmentId": {
           "type": "string"