ai/live: Error handling and retries for event subscriptions.

[j0sh]

Added this after noticing a number of errors in the prod logs for the event subscriber along the lines of

Channel exists but sequence does not: requested 71 latest 71

This is harmless and usually happens at the very end of the stream when a publisher cancels a segment just before closing the entire channel, but I thought it would be best to make the overall behavior a little more robust.

Specifically:

• Exits immediately on EOS / StreamNotFound
• Retries 5 times max
• Retry counter resets to zero after each successful read
• 300ms sleep in between each retry
• Stops the pipeline if max retries hit

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 33.72469%. Comparing base (cfc7b6e) to head (4fa7bfd).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
server/ai_live_video.go	0.00000%	26 Missing ⚠️
trickle/trickle_server.go	0.00000%	3 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3338         +/-   ##
===================================================
- Coverage   33.74545%   33.72469%   -0.02076%     
===================================================
  Files            141         141                 
  Lines          37368       37391         +23     
===================================================
  Hits           12610       12610                 
- Misses         24038       24061         +23     
  Partials         720         720                 

Files with missing lines	Coverage Δ
trickle/trickle_server.go	0.00000% <0.00000%> (ø)
server/ai_live_video.go	0.00000% <0.00000%> (ø)

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Files with missing lines	Coverage Δ
trickle/trickle_server.go	0.00000% <0.00000%> (ø)
server/ai_live_video.go	0.00000% <0.00000%> (ø)