fix/bidirectional stream shutdown

[pulimsr]

Issue #3911 & #3917

Aws::ShutdownAPI() hung indefinitely after InvokeModelWithBidirectionalStreamAsync() because a reference cycle (stream → signing callback → ctx → httpRequest → body → stream) kept an HTTP/2 connection alive, blocking CleanupCrt in ~ClientBootstrap.

• Fix (eddb695, @sbiscigl): release m_stream / m_connection in HttpWriteDataStreamBuf::WaitForStreamComplete(); drop the eventEncoderStream self-capture in codegen.
• Regression test: Nova Sonic bidi session in the bedrock-runtime integration suite, with a 60s shutdown watchdog in RunTests.cpp that turns a future hang into SIGABRT on any platform.
• Transcribe Streaming Low Speed Limit Registers a low speed limit on the CRT http client so connections will drop when data is not being transferred.

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