fix: treat a single image value as one image instead of iterating it

[betacatsling]

Summary

Fixes #734.

_copy_images iterates whatever it is given. When a caller passes a single image value instead of a list — e.g. client.chat(..., messages=[{'role': 'user', 'images': 'aW1hZ2U='}]) or client.generate(..., images='aW1hZ2U=') — the str/bytes value is a Sequence, so it is iterated one character/byte at a time and each element is wrapped in its own Image. A single base64 string becomes dozens of one-character images and serialization fails with a cryptic PydanticSerializationError: ValueError: Invalid image data, expected base64 string or path to image file; a file path string similarly explodes into per-character paths.

This wraps a bare str, bytes, os.PathLike, or Image in a single-element list before iterating, so one image value produces one image in the request. List/tuple input is unchanged.

Validation

• New regression tests in tests/test_client.py (pytest-httpserver asserts the exact request body sent):
  • test_client_chat_single_image_not_in_list (parametrized over base64 string and raw bytes)
  • test_client_generate_single_image_not_in_list
• Before this change: all three fail — each character/byte becomes a separate Image and request serialization raises PydanticSerializationError.
• After: pytest tests/test_client.py -k 'single_image or chat_images' — 8 passed; full suite pytest tests/ — 100 passed.
• ruff check and ruff format --check pass on changed files.