refactor: use std::filesystem to refactor file_helper to support utf8 path on Windows

[JalinWang]

Summary

This PR refactors file/path handling to consistently support UTF-8 paths, with a focus on Windows compatibility.

fix: #339

What Changed

• rewrote ailego::FileHelper around std::filesystem
• added UTF-8 path conversion helpers for Windows (UTF-8 <-> wide string)
• switched Windows file open/create paths from narrow Win32 APIs to wide-char APIs
• introduced PathJoin, OpenIfstream, and OpenOfstream helpers for UTF-8-safe path and stream handling
• updated DB path builders and call sites to stop manually concatenating "/" into paths
• updated version manager recovery and file IO to preserve UTF-8 filenames
• enabled RocksDB Windows UTF-8 filename support in CMake
• tightened a small default init in Version and adjusted a few tests/assertions around path behavior

Tests

Added coverage for:

• UTF-8 directory creation/removal
• nested UTF-8 paths
• file create/read/write with UTF-8 names
• rename across UTF-8 paths
• UTF-8-safe ifstream / ofstream
• collection create/open/flush/destroy under UTF-8 paths
• version manager path handling updates

[JalinWang]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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[feihongxu0824]

LGTM