Cleaned up sha256.rs and added test cases

[joeyschmoe]

This PR updates the SHA-256 hash function with a fixed-size 32-byte array and adds test cases.

In this version:
The copy_from_slice method is used to convert the hash result (which is always a 32-byte digest) into a fixed-size array. This method is safe and avoids using unwrap() or dealing with Result types for conversions. unwrap() is unsafe because it risks task failure and thus is not a composable error handling mechanism.
SHA-256 always produces a 32-byte digest, so an assertion to check the output length was redundant. Removing it simplifies error handling.

• There was only one test case for the hash, so I added two new test cases, one for b"hello world" and one for b"", to verify consistency.

Edit:
Error handling was unnecessary, so this change now only adds test cases for the hash.

[CLAassistant]

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