[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/_bootstrap.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import base64
 import contextlib
 import importlib.util
 import sys
@@ -95,6 +96,11 @@ def _generated_sample_png_bytes() -> bytes:
     )
 
 
+def generated_sample_image_data_url() -> str:
+    encoded = base64.b64encode(_generated_sample_png_bytes()).decode("ascii")
+    return f"data:image/png;base64,{encoded}"
+
+
 @contextlib.contextmanager
 def temporary_sample_image_path() -> Iterator[Path]:
     with tempfile.TemporaryDirectory(prefix="codex-python-example-image-") as temp_root: