[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/examples/07_image_and_text/sync.py

@@ -5,20 +5,20 @@ _EXAMPLES_ROOT = Path(__file__).resolve().parents[1]
 if str(_EXAMPLES_ROOT) not in sys.path:
     sys.path.insert(0, str(_EXAMPLES_ROOT))
 
-from _bootstrap import ensure_local_sdk_src, runtime_config
+from _bootstrap import ensure_local_sdk_src, generated_sample_image_data_url, runtime_config
 
 ensure_local_sdk_src()
 
 from openai_codex import Codex, ImageInput, TextInput
 
-REMOTE_IMAGE_URL = "https://raw.githubusercontent.com/github/explore/main/topics/python/python.png"
+IMAGE_DATA_URL = generated_sample_image_data_url()
 
 with Codex(config=runtime_config()) as codex:
     thread = codex.thread_start(model="gpt-5.4", config={"model_reasoning_effort": "high"})
     result = thread.turn(
         [
             TextInput("What is in this image? Give 3 bullets."),
-            ImageInput(REMOTE_IMAGE_URL),
+            ImageInput(IMAGE_DATA_URL),
         ]
     ).run()