* fix(proxy): use return value from CustomLogger.async_post_call_success_hook
Previously the return value was ignored for CustomLogger callbacks,
preventing users from modifying responses. Now the return value is
captured and used to replace the response (if not None), consistent
with CustomGuardrail and streaming iterator hook behavior.
Fixes issue with custom_callbacks not being able to inject data into
LLM responses.
* fix(proxy): also fix async_post_call_streaming_hook to use return value
Previously the streaming hook only used return values that started with
"data: " (SSE format). Now any non-None return value is used, consistent
with async_post_call_success_hook and streaming iterator hook behavior.
Added tests for streaming hook transformation.
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Co-authored-by: Gabriele Michelli <michelligabriele0@gmail.com>
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