[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/_run.py

@@ -10,7 +10,7 @@ from .generated.v2_all import (
     ThreadItem,
     ThreadTokenUsage,
     ThreadTokenUsageUpdatedNotification,
-    Turn as AppServerTurn,
+    Turn,
     TurnCompletedNotification,
     TurnError,
     TurnStatus,
@@ -55,7 +55,7 @@ def _final_assistant_response_from_items(items: list[ThreadItem]) -> str | None:
     return last_unknown_phase_response
 
 
-def _raise_for_failed_turn(turn: AppServerTurn) -> None:
+def _raise_for_failed_turn(turn: Turn) -> None:
     if turn.status != TurnStatus.failed:
         return
     if turn.error is not None and turn.error.message: