Throw explicit InvalidImageContentException when BMP BPP is invalid.

[JimBobSquarePants]

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Description

This pull request strengthens validation for BMP image decoding and adds a new test to ensure invalid BMP files are handled correctly. The main change is enforcing that only valid bitsPerPixel values are accepted, preventing malformed BMP images from being processed.

Validation improvements in BMP decoding:

• Added a check in BmpDecoderCore.Decode to ensure bitsPerPixel is one of the valid values (1, 2, 4, 8, 16, 24, or 32). If not, decoding throws an InvalidImageContentException.
• Refactored switch statements in BmpDecoderCore.Decode to use the validated bitsPerPixel variable instead of accessing the header directly, improving code clarity and consistency. [1] [2]

Testing enhancements:

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