refactor(metadata): change uploading_count type to u64

[gaius-qi]

Description

This pull request updates the type of the uploading_count field from i64 to u64 in several structs within dragonfly-client-storage/src/metadata.rs. This change ensures that upload counts, which should never be negative, are now represented as unsigned integers.

Type consistency and correctness:

• Changed the uploading_count field from i64 to u64 in the Task struct to prevent negative upload counts.
• Changed the uploading_count field from i64 to u64 in the CacheTask struct for the same reason.
• Changed the (deprecated) uploading_count field from i64 to u64 in the Piece struct.

Related Issue

Motivation and Context

Screenshots (if appropriate)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.16%. Comparing base (1ef9f88) to head (0f306f6).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1526      +/-   ##
==========================================
+ Coverage   48.12%   48.16%   +0.03%     
==========================================
  Files          70       70              
  Lines       17987    17987              
==========================================
+ Hits         8657     8664       +7     
+ Misses       9330     9323       -7     

Files with missing lines	Coverage Δ
dragonfly-client-storage/src/metadata.rs	56.94% <ø> (ø)

... and 1 file with indirect coverage changes

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[chlins]

lgtm

[fcgxz2003]

LGTM