[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/src/openai_codex/_inputs.py

@@ -14,7 +14,7 @@ class TextInput:
 
 @dataclass(slots=True)
 class ImageInput:
-    """Remote image URL supplied as turn input."""
+    """Image data URL supplied as turn input."""
 
     url: str