ai/live: terminate streams signaled by heartbeat response

[gioelecerati]

sendHeartbeat was only inspecting the HTTP status code and discarding the response body on 2xx, so the terminatedStreamIds list from the gateway was silently dropped and affected streams kept heartbeating forever.

What does this pull request do? Explain your changes. (required)
Parse the body, and for each returned stream id look up the matching LivePipeline and invoke its kickInput func (stored as StopStream on the pipeline at registerControl time). That triggers the existing cleanup chain: segmenter cancel → cleanupControl → orchCancel.

Specific updates (required)

How did you test each of these updates (required)

Does this pull request close any open issues?

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

❌ Patch coverage is 6.66667% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.85752%. Comparing base (9e68815) to head (9558bda).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
server/ai_mediaserver.go	7.50000%	34 Missing and 3 partials ⚠️
server/ai_live_video.go	0.00000%	5 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3902         +/-   ##
===================================================
- Coverage   32.87450%   32.85752%   -0.01698%     
===================================================
  Files            171         171                 
  Lines          42063       42103         +40     
===================================================
+ Hits           13828       13834          +6     
- Misses         27194       27225         +31     
- Partials        1041        1044          +3     

Files with missing lines	Coverage Δ
core/livepeernode.go	75.16779% <ø> (ø)
server/ai_live_video.go	0.00000% <0.00000%> (ø)
server/ai_mediaserver.go	6.68693% <7.50000%> (+0.04136%)	⬆️

... and 1 file with indirect coverage changes

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Files with missing lines	Coverage Δ
core/livepeernode.go	75.16779% <ø> (ø)
server/ai_live_video.go	0.00000% <0.00000%> (ø)
server/ai_mediaserver.go	6.68693% <7.50000%> (+0.04136%)	⬆️

... and 1 file with indirect coverage changes

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