Raise ValueError instead of asserting on FTEX format count

[ritsth]

FtexImageFile._open used assert format_count == 1 to reject multi-format files. That raises AssertionError, which is not one of the exceptions Pillow documents for unparseable files, so callers catching UnidentifiedImageError or ValueError do not catch it; and under python -O the assert is stripped, so the file is parsed as if valid.

This raises a ValueError instead, matching the existing "Invalid texture compression format" check in the same plugin. Added a regression test that patches the format count in a valid file, mirroring test_invalid_texture.

Used an AI assistant to find this; I reviewed and tested the change myself.

[radarhere]

So are you saying you actually encountered a multi-format file in the wild?