fix(voice): mark stream as ended

[nyapat]

Please describe the changes this PR makes and why it should be merged:

closes #8778

example:

• subscribe to user.
• user starts speaking (pipeline is: check if subscription exists, if stream exists, voice packets are decrypted, and pushed to stream)
• user stops speaking (specifically when endbehavior aftersilence or afterinactivity determines its 'stopped', push null to mark end)
• user starts speaking (subscription exists: yep, stream exists: yep, push to stream!)
• error: pushing after stream EOF

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

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[vladfrangu]

If possible, can we get a test script for this too? 🙏

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38.21%. Comparing base (40fbd82) to head (f90cb28).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10455      +/-   ##
==========================================
+ Coverage   38.18%   38.21%   +0.02%     
==========================================
  Files         239      239              
  Lines       15563    15567       +4     
  Branches     1376     1379       +3     
==========================================
+ Hits         5943     5949       +6     
+ Misses       9605     9603       -2     
  Partials       15       15              

Flag	Coverage Δ
voice	70.77% <100.00%> (+0.16%)	⬆️

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[nyapat]

drafted because putting tests was a good idea and i found undesirable behaviour (particularly annoying because the tests & actual context of the function don't align properly)

[vladfrangu]

Considering voice is an "if may work" basis, and assuming you've tested this with actual discord behavior, LGTM