[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_public_api_runtime_behavior.py

@@ -52,7 +52,7 @@ def test_codex_init_failure_closes_client(monkeypatch: pytest.MonkeyPatch) -> No
             self._closed = True
             closed.append(True)
 
-    monkeypatch.setattr(public_api_module, "AppServerClient", FakeClient)
+    monkeypatch.setattr(public_api_module, "CodexClient", FakeClient)
 
     with pytest.raises(RuntimeError, match="missing required metadata"):
         Codex()