[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/src/openai_codex/_message_router.py

@@ -4,7 +4,7 @@ import queue
 import threading
 from collections import deque
 
-from .errors import AppServerError, map_jsonrpc_error
+from .errors import CodexError, map_jsonrpc_error
 from .generated.notification_registry import notification_turn_id
 from .generated.v2_all import AccountLoginCompletedNotification
 from .models import JsonValue, Notification, UnknownNotification
@@ -136,7 +136,7 @@ class MessageRouter:
                     )
                 )
             else:
-                waiter.put(AppServerError("Malformed JSON-RPC error response"))
+                waiter.put(CodexError("Malformed JSON-RPC error response"))
             return
 
         waiter.put(msg.get("result"))