Backport: V3 - Throw explicit InvalidImageContentException when BMP BPP is invalid.

[JimBobSquarePants]

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Description

This pull request improves the robustness of the BMP decoder by adding stricter validation for the bitsPerPixel field and introducing a corresponding unit test. The main change ensures that invalid or unsupported bitsPerPixel values are properly detected and handled, preventing potential errors when decoding malformed BMP files.

Validation improvements in BMP decoding:

• Refactored the Decode method in BmpDecoderCore.cs to use a local bitsPerPixel variable for clarity and consistency throughout the code.
• Added a validation check in Decode to throw an InvalidImageContentException when bitsPerPixel is not in the supported range (>0 and <=8, 16, 24, 32), preventing decoding of invalid BMP files.

Testing enhancements:

• Added a unit test BmpDecoder_ThrowsException_Issue3067 in BmpDecoderTests.cs to verify that decoding a BMP file with bitsPerPixel = 0 correctly throws an InvalidImageContentException.