test: add unit test for TCP client and server

[ftljk]

Description

This PR introduces comprehensive unit tests for the TCP client and server components to improve code coverage and ensure reliability of network communication functionality.

Related Issue

dragonflyoss/design#8

Motivation and Context

Screenshots (if appropriate)

[codecov]

Codecov Report

❌ Patch coverage is 96.98068% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.33%. Comparing base (50a0195) to head (fa34102).

Files with missing lines	Patch %	Lines
dragonfly-client-storage/src/client/mod.rs	97.05%	13 Missing ⚠️
dragonfly-client-storage/src/client/quic.rs	0.00%	6 Missing ⚠️
dragonfly-client-storage/src/client/tcp.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1383      +/-   ##
==========================================
+ Coverage   48.89%   51.33%   +2.44%     
==========================================
  Files          68       69       +1     
  Lines       16848    17681     +833     
==========================================
+ Hits         8237     9077     +840     
+ Misses       8611     8604       -7     

Files with missing lines	Coverage Δ
dragonfly-client-storage/src/server/tcp.rs	82.11% <100.00%> (+82.11%)	⬆️
dragonfly-client/src/resource/piece_downloader.rs	22.36% <ø> (ø)
dragonfly-client-storage/src/client/quic.rs	0.00% <0.00%> (ø)
dragonfly-client-storage/src/client/tcp.rs	0.00% <0.00%> (ø)
dragonfly-client-storage/src/client/mod.rs	97.05% <97.05%> (ø)

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.