ai/live: Add unit tests for MediaReader / MediaWriter

[j0sh]

Mostly done to increase confidence around #3447

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 30.37938%. Comparing base (d543d17) to head (20cb3da).
Report is 1 commits behind head on master.

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3466         +/-   ##
===================================================
+ Coverage   30.31420%   30.37938%   +0.06518%     
===================================================
  Files            151         151                 
  Lines          44494       44494                 
===================================================
+ Hits           13488       13517         +29     
+ Misses         30219       30187         -32     
- Partials         787         790          +3     

see 3 files with indirect coverage changes

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see 3 files with indirect coverage changes

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