[codex] Rename Python SDK AppServerConfig to CodexConfig (#24800)

## Why

`AppServerConfig` is exported as part of the ergonomic Python SDK
surface and passed to `Codex(...)` and `AsyncCodex(...)`. That name
exposes the underlying app-server transport at the same layer where
users are configuring the Codex client. `CodexConfig` makes the common
callsite read naturally and names the object it configures.

## What changed

- Renamed the public configuration dataclass from `AppServerConfig` to
`CodexConfig`.
- Updated `Codex`, `AsyncCodex`, and the transport clients to accept
`CodexConfig`.
- Updated binary-resolution messages, package exports, docs, examples,
and related coverage to use the new public name.

## API impact

```python
from openai_codex import Codex, CodexConfig

with Codex(config=CodexConfig(codex_bin="/path/to/codex")) as codex:
    ...
```

Callers should now import and construct `CodexConfig`; `AppServerConfig`
is no longer part of the Python SDK surface.

## Validation

- `uv run --frozen --extra dev ruff check src/openai_codex scripts
examples tests`
- Tests are deferred to online CI for this PR.

sdk/python/tests/test_app_server_streaming.py

@@ -114,7 +114,7 @@ def test_async_stream_routes_text_deltas_and_completion(tmp_path) -> None:
 
 
 def test_low_level_sync_stream_text_uses_real_turn_routing(tmp_path) -> None:
-    """AppServerClient.stream_text should stream through a real app-server turn."""
+    """CodexClient.stream_text should stream through a real app-server turn."""
     with AppServerHarness(tmp_path) as harness:
         harness.responses.enqueue_sse(
             streaming_response("low-sync-stream", "msg-low-sync-stream", ["fir", "st"])