[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/examples/13_model_select_and_turn_params/async.py

@@ -13,12 +13,12 @@ import asyncio
 
 from openai_codex import (
     AsyncCodex,
+    Sandbox,
 )
 from openai_codex.types import (
     Personality,
     ReasoningEffort,
     ReasoningSummary,
-    SandboxPolicy,
 )
 
 REASONING_RANK = {
@@ -67,13 +67,6 @@ OUTPUT_SCHEMA = {
     "additionalProperties": False,
 }
 
-SANDBOX_POLICY = SandboxPolicy.model_validate(
-    {
-        "type": "readOnly",
-        "access": {"type": "fullAccess"},
-    }
-)
-
 
 async def main() -> None:
     async with AsyncCodex(config=runtime_config()) as codex:
@@ -106,7 +99,7 @@ async def main() -> None:
             model=selected_model.model,
             output_schema=OUTPUT_SCHEMA,
             personality=Personality.pragmatic,
-            sandbox_policy=SANDBOX_POLICY,
+            sandbox=Sandbox.read_only,
             summary=ReasoningSummary.model_validate("concise"),
         )
         second = await second_turn.run()

sdk/python/examples/13_model_select_and_turn_params/sync.py

@@ -11,12 +11,12 @@ ensure_local_sdk_src()
 
 from openai_codex import (
     Codex,
+    Sandbox,
 )
 from openai_codex.types import (
     Personality,
     ReasoningEffort,
     ReasoningSummary,
-    SandboxPolicy,
 )
 
 REASONING_RANK = {
@@ -65,14 +65,6 @@ OUTPUT_SCHEMA = {
     "additionalProperties": False,
 }
 
-SANDBOX_POLICY = SandboxPolicy.model_validate(
-    {
-        "type": "readOnly",
-        "access": {"type": "fullAccess"},
-    }
-)
-
-
 with Codex(config=runtime_config()) as codex:
     models = codex.models(include_hidden=True)
     selected_model = _pick_highest_model(models.data)
@@ -102,7 +94,7 @@ with Codex(config=runtime_config()) as codex:
         model=selected_model.model,
         output_schema=OUTPUT_SCHEMA,
         personality=Personality.pragmatic,
-        sandbox_policy=SANDBOX_POLICY,
+        sandbox=Sandbox.read_only,
         summary=ReasoningSummary.model_validate("concise"),
     ).run()