fix: Ensure multiplex is destroyed

[FrederikBolding]

Discovered while investigating a memory leak, does not seem to be a direct fix though we should be destroying all streams that are piped.

---

Note

Ensures the multiplexed stream is properly tracked and destroyed during snap termination.

• Extends JobStreams to include mux: ObjectMultiplex and renames _connection to connection
• Returns mux and connection from #initStreams and includes them in job.streams so termination destroys them with other streams

^{Written by Cursor Bugbot for commit 4fa79ec. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.34%. Comparing base (4aaa4c3) to head (4fa79ec).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3795   +/-   ##
=======================================
  Coverage   98.34%   98.34%           
=======================================
  Files         422      422           
  Lines       12059    12059           
  Branches     1872     1872           
=======================================
  Hits        11859    11859           
  Misses        200      200           

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