test: add comprehensive roundtrip tests and fix leading zeros bug

[sumanjeet0012]

Fixes #40

Description

This pull request adds comprehensive round-trip tests with random data to verify that all encodings in py-multibase correctly handle various data shapes, including leading zero bytes and extreme sizes. In addition, it fixes a bug where BaseStringConverter was incorrectly dropping leading zeros upon encoding/decoding.

Changes

• Created tests/test_roundtrip.py with random data generators covering all 24 supported encodings (e.g. base2, base8, base10, base16, base32z, base36, base58btc, etc.).
• The tests check data integrity on edge-cases such as random sizes, all zeros (b'\x00\x00'), all ones (b'\xff\xff'), and leading zeroes (b'\x00hello').
• Fixed BaseStringConverter.encode and decode in multibase/converters.py to correctly preserve leading zero bytes manually, as int.from_bytes naturally truncates them.
• Fixed Base16StringConverter.decode in multibase/converters.py to use bytes.fromhex() instead of relying on the integer representation decoding.