Fix ImagePalette colors for raw palettes

[nightcityblade]

Fixes #9820.

Changes proposed in this pull request:

• Convert raw palette data through Pillow's existing palette conversion path before constructing ImagePalette.colors.
• Preserve the correct channel order and entry width for raw modes such as BGRX.
• Add a regression test covering the five-color BGRX palette from the issue.

Tests:

• python -m pytest Tests/test_imagepalette.py Tests/test_file_bmp.py Tests/test_image_getpalette.py Tests/test_image_putpalette.py -q (64 passed)
• ruff check --no-fix src/PIL/ImagePalette.py Tests/test_imagepalette.py
• ruff format --check src/PIL/ImagePalette.py Tests/test_imagepalette.py
• black --check src/PIL/ImagePalette.py Tests/test_imagepalette.py
• mypy --python-executable .venv/bin/python src/PIL/ImagePalette.py Tests/test_imagepalette.py

This change was prepared by an AI coding agent. I reviewed the focused diff and verified the issue reproducer and tests locally.

[radarhere]

Suggested change

	bytes.fromhex("00000000ffffff000000ff0000ff0000ff000000"),
	"BGRX", (0, 0, 0, 0, 255, 255, 255, 0, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 0)

This makes it much easier to read, swap around the channels in your head, and understand how the BGRX values are transformed into the expected RGB values.

[nightcityblade]

Applied in b3bbfe5. The test now uses explicit channel values as suggested, and the core conversion normalizes sequence input to bytes. Local validation: all 64 focused palette/BMP/getpalette/putpalette tests passed; Ruff check and format passed on both changed files. Thanks.