[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/app_server_harness.py

@@ -10,7 +10,7 @@ from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 from pathlib import Path
 from typing import Any
 
-from openai_codex import AppServerConfig
+from openai_codex import CodexConfig
 
 Json = dict[str, Any]
 
@@ -225,9 +225,9 @@ class AppServerHarness:
         shutil.rmtree(self.codex_home, ignore_errors=True)
         shutil.rmtree(self.workspace, ignore_errors=True)
 
-    def app_server_config(self) -> AppServerConfig:
+    def app_server_config(self) -> CodexConfig:
         """Build SDK config for an isolated pinned-runtime app-server process."""
-        return AppServerConfig(
+        return CodexConfig(
             cwd=str(self.workspace),
             env={
                 "CODEX_HOME": str(self.codex_home),