[codex] Add friendly Python SDK sandbox presets (#24772)

## Why

The Python SDK currently exposes sandbox selection differently depending
on where it is used: thread lifecycle methods accept `SandboxMode`,
while turns accept the lower-level `SandboxPolicy` shape. For the common
case of choosing an access level, that leaks app-server wire details
into otherwise straightforward SDK usage.

This makes the common path explicit and discoverable: callers choose a
named sandbox preset once, using the same keyword on threads and turns.
The preset name `workspace_write` also makes the granted capability
clear at the callsite.

## What changed

- Added a root-level `Sandbox` enum with documented presets:
  - `Sandbox.read_only`: read files without allowing writes.
- `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.
- Documented that omitting `sandbox=` delegates to app-server's
configured default, while explicit turn overrides remain sticky for
subsequent turns.
- Updated sync and async thread lifecycle and turn APIs to consistently
accept `sandbox=Sandbox...`, translating to the existing app-server
thread and turn representations internally.
- Updated the public API artifact generator so regenerated SDK wrappers
retain the friendly enum shape.
- Replaced low-level policy construction in Python docs, examples, and
the walkthrough notebook with the preset API.
- Added focused coverage for root exports, method signatures,
preset-to-wire mapping, and rejection of raw string sandbox inputs.

## API impact

High-level turn calls now use `sandbox=` instead of `sandbox_policy=`:

```python
from openai_codex import Codex, Sandbox

with Codex() as codex:
    thread = codex.thread_start(sandbox=Sandbox.workspace_write)
    result = thread.run("Review the diff only.", sandbox=Sandbox.read_only)
```

`thread_start(...)` already defaults to `ApprovalMode.auto_review`, so
normal writable usage is concise:

```python
with Codex() as codex:
    thread = codex.thread_start(sandbox=Sandbox.workspace_write)
    thread.run("Update the files in this workspace.")
```

With that combination, edits inside `cwd` and configured writable roots
run within the workspace-write sandbox. Operations that require
approval, such as edits outside those roots, are routed through auto
review. When `sandbox=` is omitted, app-server resolves its configured
default. A sandbox supplied to `run(...)` or `turn(...)` applies to that
turn and subsequent turns.

## Test coverage

- `sdk/python/tests/test_public_api_signatures.py` covers the public
export and parameter names, including the default approval mode.
- `sdk/python/tests/test_public_api_runtime_behavior.py` covers preset
mappings to the existing wire types and raw string rejection.

sdk/python/README.md

@@ -23,12 +23,12 @@ when you intentionally want to run against a specific local app-server binary.
 ## Quickstart
 
 ```python
-from openai_codex import Codex
+from openai_codex import Codex, Sandbox
 
 with Codex() as codex:
     # Call login_api_key(...) first when this app-server session is not
     # already authenticated.
-    thread = codex.thread_start(model="gpt-5")
+    thread = codex.thread_start(model="gpt-5", sandbox=Sandbox.workspace_write)
     result = thread.run("Say hello in one sentence.")
     print(result.final_response)
     print(len(result.items))
@@ -38,6 +38,29 @@ with Codex() as codex:
 `final_response` is `None` when the turn completes without a final-answer or
 phase-less assistant message item.
 
+## Sandbox
+
+Use the same enum when creating a thread or changing its sandbox for a turn:
+
+```python
+from openai_codex import Codex, Sandbox
+
+with Codex() as codex:
+    thread = codex.thread_start(sandbox=Sandbox.workspace_write)
+    thread.run("Make the requested change.")
+    review = thread.run("Review the diff only.", sandbox=Sandbox.read_only)
+```
+
+Available presets:
+
+- `Sandbox.read_only`: read files without allowing writes.
+- `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 sandbox
+passed to `run(...)` or `turn(...)` applies to that turn and subsequent turns
+on the thread.
+
 ## Login
 
 Use the auth helper that matches your app: