[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/docs/getting-started.md

@@ -99,7 +99,7 @@ Available presets:
 - `Sandbox.workspace_write`: the normal default for projects with a recorded trust decision; read files and write inside the workspace and configured writable roots.
 - `Sandbox.full_access`: run without filesystem access restrictions.
 
-When `sandbox=` is omitted, app-server uses its configured default. A turn
+When `sandbox=` is omitted, Codex uses its configured default. A turn
 override also becomes the sandbox for subsequent turns on that thread.
 
 ## 5) Continue the same thread (multi-turn)
@@ -150,9 +150,9 @@ with Codex() as codex:
     print(result.final_response)
 ```
 
-## 8) Public app-server types
+## 8) Public Codex protocol types
 
-The convenience wrappers live at the package root. Public app-server value and
+The convenience wrappers live at the package root. Public Codex protocol value and
 event types live under:
 
 ```python