[6/8] Add high-level Python SDK approval mode (#21910)

## Why

The high-level SDK should expose the approval behavior it actually
supports instead of leaking generated app-server routing fields. New
work should have two clear choices: default auto review, or explicitly
deny escalated permission requests. Existing threads and subsequent
turns should preserve their current approval behavior unless the caller
passes an override.

## What

- Add the public `ApprovalMode` enum with `auto_review` and `deny_all`.
- Default new thread creation to `ApprovalMode.auto_review`.
- Preserve existing approval settings by default for resume, fork, run,
and turn helpers.
- Remove raw `approval_policy` / `approvals_reviewer` kwargs from
high-level SDK wrappers.
- Update generated wrapper output, docs, examples, notebooks, and tests
for the high-level approval mode API.

## Stack

1. #21891 `[1/8]` Pin Python SDK runtime dependency
2. #21893 `[2/8]` Generate Python SDK types from pinned runtime
3. #21895 `[3/8]` Run Python SDK tests in CI
4. #21896 `[4/8]` Define Python SDK public API surface
5. #21905 `[5/8]` Rename Python SDK package to `openai-codex`
6. This PR `[6/8]` Add high-level Python SDK approval mode
7. #22014 `[7/8]` Add Python SDK app-server integration harness
8. #22021 `[8/8]` Add Python SDK Ruff formatting

## Verification

- Added approval-mode mapping/default tests for new threads, existing
threads, forks, resumes, and subsequent turns.

---------

Co-authored-by: Codex <noreply@openai.com>

sdk/python/notebooks/sdk_walkthrough.ipynb

@@ -246,7 +246,6 @@
     "# Cell 5b: one turn with most optional turn params\n",
     "from pathlib import Path\n",
     "from openai_codex import (\n",
-    "    AskForApproval,\n",
     "    Personality,\n",
     "    ReasoningEffort,\n",
     "    ReasoningSummary,\n",
@@ -270,7 +269,6 @@
     "    thread = codex.thread_start(model='gpt-5.4', config={'model_reasoning_effort': 'high'})\n",
     "    turn = thread.turn(\n",
     "        TextInput('Propose a safe production feature-flag rollout. Return JSON matching the schema.'),\n",
-    "        approval_policy=AskForApproval.model_validate('never'),\n",
     "        cwd=str(Path.cwd()),\n",
     "        effort=ReasoningEffort.medium,\n",
     "        model='gpt-5.4',\n",
@@ -296,7 +294,6 @@
     "# Cell 5c: choose highest model + highest supported reasoning, then run turns\n",
     "from pathlib import Path\n",
     "from openai_codex import (\n",
-    "    AskForApproval,\n",
     "    Personality,\n",
     "    ReasoningEffort,\n",
     "    ReasoningSummary,\n",
@@ -361,7 +358,6 @@
     "\n",
     "    second = thread.turn(\n",
     "        TextInput('Return JSON for a safe feature-flag rollout plan.'),\n",
-    "        approval_policy=AskForApproval.model_validate('never'),\n",
     "        cwd=str(Path.cwd()),\n",
     "        effort=selected_effort,\n",
     "        model=selected_model.model,\n",