ai/live: Another way to improve control channel cleanup.

[j0sh]

This achieves a similar effect as #3483 except perhaps more localized with fewer ad hoc cleanup calls - we just call cleanupLive once per session (although cleanupLive would perhaps be better named "stopControl" or thereabouts)

This allows us to ensure that orchestrator selection completes before beginning this control channel cleanup process, which mitigates any risk of ingest and control (the two primary input sources to the runner) falling out of sync due to the same cleanup function being called from various places.

Note that the gateway WHIP handler did not have this race condition because cleanup is correctly sequenced : we wait for selection to complete, wait for ingest to complete, then clean up the control channel. This PR basically introduces a similar sequencing.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 30.64248%. Comparing base (055484e) to head (c87d679).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
server/ai_mediaserver.go	0.00000%	6 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3502         +/-   ##
===================================================
- Coverage   30.64298%   30.64248%   -0.00050%     
===================================================
  Files            151         151                 
  Lines          45087       45091          +4     
===================================================
+ Hits           13816       13817          +1     
- Misses         30463       30466          +3     
  Partials         808         808                 

Files with missing lines	Coverage Δ
server/ai_live_video.go	0.00000% <ø> (ø)
server/ai_mediaserver.go	5.22565% <0.00000%> (-0.03122%)	⬇️

... and 1 file with indirect coverage changes

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Files with missing lines	Coverage Δ
server/ai_live_video.go	0.00000% <ø> (ø)
server/ai_mediaserver.go	5.22565% <0.00000%> (-0.03122%)	⬇️

... and 1 file with indirect coverage changes

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