[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/faq.md

@@ -66,7 +66,7 @@ The presets are:
 - `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
 sandbox override applies to that turn and subsequent turns.
 
 ## Why only `thread_start(...)` and `thread_resume(...)`?
@@ -86,7 +86,7 @@ Common causes:
 
 - published runtime package (`openai-codex-cli-bin`) is not installed
 - local `codex_bin` override points to a missing file
-- app-server version older than the SDK schema
+- installed Codex runtime version older than the SDK schema
 
 ## Why does a turn "hang"?