fix: treat a bare str/bytes image as one image, not per-character

[er-s-an]

Summary

Fixes #734.

_copy_images iterates its argument. A bare str / bytes / path-like / Image is a Sequence, so it was iterated character/byte-wise and each fragment became its own Image. That produced cryptic serialization errors for chat message images and exploded file paths for generate(..., images=...).

Wrap a single image value in a one-element list before iterating. List/tuple input is unchanged.

Validation

python -m venv .venv && .venv/bin/pip install -e . pytest pytest-anyio pytest-httpserver
.venv/bin/pytest tests/test_client.py -k 'single_image or chat_images or generate_with_image' -q
# 9 passed
.venv/bin/pytest tests/ -q
# 118 passed

New regression tests:

• test_client_chat_single_image_not_in_list (parametrized over base64 string and raw bytes)
• test_client_generate_single_image_not_in_list