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

	image = Image.new("P", (1, 1))
	image.putpalette(palette, self.rawmode)
	converted_palette = image.getpalette(self.mode)
	assert converted_palette is not None
	palette = converted_palette
	im = Image.core.new("P", (0, 0))
	im.putpalette(self.mode, self.rawmode, palette)
	palette = im.getpalette(self.mode, self.mode)

[nightcityblade]

Applied in 84571ab. The raw palette now uses the core image conversion path from your suggestion. The focused palette/BMP/getpalette/putpalette suite passes (64 tests), and Ruff check/format both pass.