fix: prevent proxy accept tight loop and connection leaks

[zx06]

Summary

• Fix accept tight loop in proxy by adding exponential backoff on listener errors.
• Fix connection and goroutine leaks by properly closing counterpart connection upon EOF or error in bidirectional copy.
• Add unit tests verifying immediate connection cleanup on remote/local disconnect.

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.21%. Comparing base (ed3f8a8) to head (93bcfc0).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/proxy/proxy.go	93.93%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   82.16%   82.21%   +0.04%     
==========================================
  Files          62       62              
  Lines        5972     5983      +11     
==========================================
+ Hits         4907     4919      +12     
+ Misses        801      798       -3     
- Partials      264      266       +2     

Flag	Coverage Δ
e2e	28.65% <ø> (ø)
integration	28.65% <ø> (ø)
unittests	74.72% <93.93%> (+0.06%)	⬆️

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[sonarqubecloud]

Quality Gate failed

Failed conditions
17.1% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud