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/core/src/shell_snapshot.rs

@@ -76,7 +76,9 @@ impl ShellSnapshot {
         }
 
         let shell = environment.shell.clone()?;
-        let cwd = environment.cwd().clone();
+        // TODO(anp): Migrate shell snapshot creation to accept PathUri and defer native
+        // conversion to the spawned shell process.
+        let cwd = environment.cwd().to_abs_path().ok()?;
         Self::build_for_cwd(Arc::clone(config), cwd, shell).await
     }