ai/live: Fix early EOFs in media reader

[j0sh]

Sometimes if the reader is slow to pull data (eg, during a retry) and the buffer grows larger than a read then closes, the reader may incorrectly signal EOF and return incomplete data. Don't do that.

The practical consequence here is we would sometime write truncated segments to the server, since the trickle publish is serviced by the segment reader which is being fixed here

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 34.03496%. Comparing base (abd997d) to head (6a68935).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
media/rw.go	0.00000%	4 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3314         +/-   ##
===================================================
- Coverage   34.03772%   34.03496%   -0.00276%     
===================================================
  Files            141         141                 
  Lines          37006       37009          +3     
===================================================
  Hits           12596       12596                 
- Misses         23691       23694          +3     
  Partials         719         719                 

Files with missing lines	Coverage Δ
media/rw.go	0.00000% <0.00000%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update abd997d...6a68935. Read the comment docs.

Files with missing lines	Coverage Δ
media/rw.go	0.00000% <0.00000%> (ø)