[WIP] fix: correct callback event firing in streaming loops

[bhimrazy]

What does this PR do?

Fixes #735.

The sync streaming loop fired AFTER_PREDICT twice and never fired the decode-request events.

While fixing it, a second issue surfaced: predict and encode_response return generators, so no work has happened when they return — it all runs interleaved as the stream is consumed. Firing the "after" events at that point meant PredictionTimeLogger reported Prediction took 0.00 seconds on every streaming request.

So all three streaming paths now fire:

BEFORE_DECODE_REQUEST → AFTER_DECODE_REQUEST → BEFORE_PREDICT
  → BEFORE_ENCODE_RESPONSE → [stream consumed] → AFTER_PREDICT → AFTER_ENCODE_RESPONSE

each exactly once.

Note: the async and BatchedStreamingLoop changes go beyond the reported issue — same bug, but they shift event timing for existing users.

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Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85%. Comparing base (4c9fecc) to head (6146f25).
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Additional details and impacted files

@@         Coverage Diff         @@
##           main   #736   +/-   ##
===================================
- Coverage    85%    85%   -0%     
===================================
  Files        39     39           
  Lines      3285   3286    +1     
===================================
- Hits       2781   2780    -1     
- Misses      504    506    +2     

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