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 50.44%. Comparing base (6cbe248) to head (669104e).

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #10455       +/-   ##
===========================================
+ Coverage   38.01%   50.44%   +12.43%     
===========================================
  Files         239      227       -12     
  Lines       15471    20742     +5271     
  Branches     1353     1305       -48     
===========================================
+ Hits         5881    10463     +4582     
- Misses       9575    10267      +692     
+ Partials       15       12        -3     

Flag	Coverage Δ
voice	72.39% <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