[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/README.md

@@ -72,7 +72,7 @@ python examples/01_quickstart_constructor/async.py
 - `06_thread_lifecycle_and_controls/`
   - thread lifecycle + control calls
 - `07_image_and_text/`
-  - remote image URL + text multimodal turn
+  - image data URL + text multimodal turn
 - `08_local_image_and_text/`
   - local image + text multimodal turn using a generated temporary sample image
 - `09_async_parity/`