fix: make HashUtil::HashBytes more platform independent

[polyominal]

https://github.com/polyominal/bustub/blob/9988c8503cc3e5107cc750c651ebeaef2efe8ce6/src/include/common/util/hash_util.h#L32-L39

The hash function uses raw bytes[i] in char. This creates a subtle bug. C++ doesn't define whether char is signed or unsigned.

For byte values over 127, this matters:

• Signed platforms: values become negative (the expected behavior)
• Unsigned platforms: values stay positive (0-255)

This bit me during HyperLogLog project:
https://github.com/polyominal/bustub/blob/9988c8503cc3e5107cc750c651ebeaef2efe8ce6/test/primer/hyperloglog_test.cpp#L58-L108

My code failed on my machine (unsigned char) but passed elsewhere (signed char).

Proposed fix: Cast to int8_t for consistent behavior.

hash = ((hash << 5) ^ (hash >> 27)) ^ static_cast<int8_t>(bytes[i]);

This should enable more machines to work. Any downsides to consider?