[sdk/python] Stop advertising HTTP image URLs (#29464)

## Summary

- use generated image data URLs in the Python SDK examples and notebook
- document HTTP and HTTPS image URLs as deprecated and recommend
`LocalImageInput`
- replace the remote-URL integration test with data-URL coverage

`ImageInput` remains available for data URLs. The SDK does not duplicate
app-server URL validation.

## Testing

- `uv run --frozen --no-sync ruff check --output-format=full .`
- `uv run --frozen --no-sync ruff format --check .`
- full Python SDK test suite with an isolated writable
`CODEX_SQLITE_HOME` (119 passed, 38 skipped)

sdk/python/notebooks/sdk_walkthrough.ipynb

@@ -103,7 +103,7 @@
    "outputs": [],
    "source": [
     "# Cell 2: imports (public only)\n",
-    "from _bootstrap import server_label\n",
+    "from _bootstrap import generated_sample_image_data_url, server_label\n",
     "from openai_codex import (\n",
     "    AsyncCodex,\n",
     "    Codex,\n",
@@ -349,14 +349,14 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "# Cell 6: multimodal with remote image\n",
-    "remote_image_url = 'https://raw.githubusercontent.com/github/explore/main/topics/python/python.png'\n",
+    "# Cell 6: multimodal with an image data URL\n",
+    "image_data_url = generated_sample_image_data_url()\n",
     "\n",
     "with Codex() as codex:\n",
     "    thread = codex.thread_start(model='gpt-5.4', config={'model_reasoning_effort': 'high'})\n",
     "    result = thread.turn([\n",
     "        TextInput('What do you see in this image? 3 bullets.'),\n",
-    "        ImageInput(remote_image_url),\n",
+    "        ImageInput(image_data_url),\n",
     "    ]).run()\n",
     "    print('status:', result.status)\n",
     "    print(result.final_response)\n"