Fix SendRequestAsync hang on Streamable HTTP SSE retry#1396

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Copilot wants to merge 8 commits intomainfrom

copilot/replace-corrupted-pr-1381

Commits on Feb 27, 2026

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Fix SendRequestAsync hang when background SSE stream delivers response before foreground transport send completes

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stephentoub
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Cancel SendToRelatedTransportAsync when response arrives via concurrent channel

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stephentoub
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Invert if block for common case, log send faults at debug level

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stephentoub
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Guard send/race block with tcs.Task.IsCompleted check to skip send when response already arrived

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stephentoub
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Simplify ObserveSendFaults: remove #if NET block, inline ContinueWith at call site

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stephentoub
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Move session ID and background GET setup before SSE processing to prevent cancellation race

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stephentoub
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Fix WhenAny race: await initialize send to completion, only race non-initialize requests

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stephentoub
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