[sdk/python] Stop advertising HTTP image URLs

[rka-oai]

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)