Close VXI-11 discovery sockets

[CAOShurong]

VXI-11 discovery creates a UDP port-mapper client for each interface but leaves their sockets to garbage collection. Calling ResourceManager("@py").list_resources() on current main produces an unclosed-socket ResourceWarning.

Keep all created clients until a finally cleanup, including clients whose broadcast fails, while preserving successful discovery results. The regression covers successful discovery and both send and receive errors.

Related to #507. Its original asyncio traceback was not reproduced; this fixes the separate socket-lifecycle problem found through the same discovery entry point.

• Executed pre-commit with no errors
• Added automated regression coverage
• Added a CHANGES entry

Validation: 309 passed, 160 skipped; strict Sphinx build and installed-wheel discovery checks passed. Physical instruments were not tested. No protocol-compliance changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.33%. Comparing base (eee01cc) to head (8ea3251).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #671      +/-   ##
==========================================
+ Coverage   54.74%   55.33%   +0.59%     
==========================================
  Files          39       39              
  Lines        6518     6549      +31     
  Branches      649      652       +3     
==========================================
+ Hits         3568     3624      +56     
+ Misses       2854     2827      -27     
- Partials       96       98       +2     

Flag	Coverage Δ
unittests	55.33% <100.00%> (+0.59%)	⬆️

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