Pin Python SDK app-server stdio to UTF-8 on Windows (#15244)

## TL;DR
Pin the Python app-server SDK subprocess pipes to UTF-8 so Windows users
on non-UTF-8 locales do not hit `UnicodeDecodeError` when the `codex`
child emits UTF-8 text.

- add `encoding="utf-8"` to the `subprocess.Popen(...)` call in
`AppServerClient.start()`
- add a focused regression test that asserts the client launches the
subprocess with UTF-8 text I/O
- validates with `python -m pytest
sdk/python/tests/test_client_rpc_methods.py
sdk/python/tests/test_client_process_launch.py
sdk/python/tests/test_public_api_runtime_behavior.py`

Fixes #14311.

sdk/python/src/codex_app_server/client.py

@@ -181,6 +181,7 @@ class AppServerClient:
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
             text=True,
+            encoding="utf-8",
             cwd=self.config.cwd,
             env=env,
             bufsize=1,