test: add unit tests for image_array_to_data_uri

[eeshsaxena]

Description

_plotly_utils.data_utils.image_array_to_data_uri converts a numpy uint8
image array into a base64 PNG/JPG data URI. It had no direct test coverage.

This adds tests/test_optional/test_utils/test_data_utils.py covering:

• greyscale (2D), RGB and RGBA arrays each producing a PNG data URI
• ValueError on out-of-range compression (-1 and 10)
• ValueError on an invalid image shape
• ValueError when requesting jpg with a non-PIL backend

The tests use the pypng backend so they do not require Pillow. No source
changes.

Verification

pytest tests/test_optional/test_utils/test_data_utils.py - 7 passed.
ruff check / ruff format --check clean.