ディスクリプターのサイズがリトルエンディアンでエンコードされていたのを修正する

[sile]

Copilot Summary

This pull request includes changes to fix the endianness issue in descriptor encoding and add tests for the encode_tag_and_size and decode_tag_and_size functions. The most important changes include updating the CHANGES.md file to reflect the fix and modifying the encoding function to correct the endianness.

Endianness fix:

• CHANGES.md: Added an entry to document the fix for the descriptor size encoding issue.
• src/descriptors.rs: Modified the encode_tag_and_size function to reverse the buffer, changing the encoding from little-endian to big-endian.

Testing:

• src/descriptors.rs: Added a test module with a test case to verify the encode_tag_and_size and decode_tag_and_size functions.