[NicolasDorier]

Problem

Related to btcpayserver/NBXplorer#578 and btcpayserver/NBXplorer#579.

SendMessageAsync checks IsConnected before invoking the outgoing filters and enqueueing the message. If the sender stops in between, it fails the messages it can see and replaces the queue. A late enqueue can land in either the already-drained queue or its replacement, leaving the returned task pending forever.

VersionHandshakeAsync awaits its sends without observing its cancellation token, so this race can also leave a handshake stuck even after cancellation.

Changes

• Keep one send queue for the lifetime of the connection and call CompleteAdding before draining it during sender shutdown.
• Fail the in-flight message and drain queued messages. A racing enqueue is either accepted before completion and drained, or rejected and its task faulted with the existing disconnected-peer exception.
• Observe the handshake cancellation token while awaiting all four handshake sends, using the existing cross-target WithCancellation helper. Cancellation stops waiting; it does not retract an already accepted send.

This fixes NBitcoin's lost-send race; NBXplorer still needs its separate fix for an old node's disconnect handler disposing a replacement connection.

Validation

The regression tests used for validation were removed from the final diff at the maintainer's request. Only the production fix is included.

• Confirmed the initial three regression cases (direct send, handshake disconnect, and cancellation of the first handshake send) time out against the original implementation.
• Before removing the tests, passed all 15 selected tests on .NET 10: six regression cases and nine existing protocol unit cases.
• Before removing the tests, passed all six regression cases with TargetFrameworkOverride=netstandard2.0, exercising the legacy cancellation and receive paths on the .NET 10 test runner.
• Built NBitcoin successfully for all four targets: net10.0, net472, netstandard2.1, and netstandard2.0.
• Did not run the full test suite or execute tests on the .NET Framework runtime.