meta: Replace doReaddir with DirHandler for batch fetching in cloneEntry

[Xuyuchao-juice]

Why this change:

The original implementation used doReaddir to load all directory entries at once, which causes performance issues and high memory pressure when cloning large directories. This leads to slow metadata operations, negative ETA values, and progress staying at 0 for extended periods.

How it's implemented:

Replace the single doReaddir call with DirHandler's List method to fetch directory entries in batches. The implementation maintains the directory-first processing strategy (directories before files) to maximize parallelism, while using incremental offset-based fetching to reduce per-query overhead and improve overall performance for large directories.

[codecov]

Codecov Report

❌ Patch coverage is 58.97436% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 22.92%. Comparing base (2d372c7) to head (ef29f65).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/meta/base.go	58.97%	26 Missing and 6 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6469       +/-   ##
===========================================
- Coverage   48.11%   22.92%   -25.20%     
===========================================
  Files         133       30      -103     
  Lines       42009    20539    -21470     
===========================================
- Hits        20213     4708    -15505     
+ Misses      19445    15298     -4147     
+ Partials     2351      533     -1818     

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[jiefenghuang]

Need to fix unit tests.