netty: Propagate initial handshake failure before close #12626
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When a handshake failure occurs before any writes are buffered on the server side, WriteBufferingAndExceptionHandler can record the failure internally but never surface it to downstream inbound handlers. This makes the original handshake error unobservable and complicates debugging and instrumentation. Propagate only the first failure via exceptionCaught, gated on the absence of a previous failure, so that the canonical error becomes observable while avoiding duplicate propagation and preserving existing close semantics.
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Handshake failures that occur before any writes are buffered can currently be lost to downstream inbound handlers. In this case, the failure is surfaced via the write / promise path, but exceptionCaught is never observed by handlers placed after WriteBufferingAndExceptionHandler.
This makes the original handshake error difficult to diagnose and inconsistent with failures that occur after buffering has started.
This change propagates the exception via fireExceptionCaught before closing the channel when handling the first failure on an active channel. Doing so preserves the original failure while the pipeline is still intact and avoids losing the
exception due to close-triggered teardown or reentrancy.
Fixes #8495