arrow

GH-40870: [C#] Update CompareValidityBuffer() to pass when unspecified final bits are not identical (#40873)


### Rationale for this change

Before fixing nanoarrow's testing JSON reader to align with other implementations and properly zero out the last few bits, integration tests failed because C#'s `CompareValidityBuffer()` was comparing the bytes of the validity buffer (including undefined final bits that are maybe not identical due to uninitialized memory or because the arrays are slices).

### What changes are included in this PR?

`CompareValidityBuffer()` now compares the memory for all except the last byte and compares the last byte bitwise.

### Are these changes tested?

They should be but I am not sure exactly where to add the test!

### Are there any user-facing changes?

No
* GitHub Issue: #40870

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Dewey Dunnington <dewey@voltrondata.com>