fix(handler): drain channels after timeout to capture final measurement

[robertodauria]

Summary

• Add a 500ms drain loop after timeout.Done() to collect any remaining in-flight measurements from senderCh/receiverCh before returning
• The sender goroutine sends a final Measure() on ctx.Done() which could be missed when the handler returned immediately on timeout
• This fixes a ~310ms gap in ElapsedTime for all MSAK tests compared to ndt7

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This change is