test: cover normalize multichannel, NaN, and validation paths

[wkentaro]

normalize only had happy-path coverage; its multichannel branch, NaN handling,
and four validation/warning guards were untested.

Summary

• Cover per-channel normalization for a 3-D image (auto and explicit min/max).
• Cover NaN propagation: a NaN stays NaN in 2-D, and a NaN in any channel
  poisons the whole pixel in a multichannel image.
• Cover the ndim, min_value.shape, and max_value.shape ValueError guards.
• Cover the inf UserWarning emitted when a min or max value is infinite.

Test plan

• uv run pytest tests/unit/_normalize_test.py (11 passed)
• full suite uv run --extra all pytest -n=auto tests (255 passed)
• uv run ruff check, ruff format --check, ty check on the changed file