Add an assertion failure if the channel's closeFuture is failed

Sources/NIOCore/AsyncChannel/AsyncChannel.swift

@@ -305,8 +305,20 @@ public struct NIOAsyncChannel<Inbound: Sendable, Outbound: Sendable>: Sendable {
         // We ignore errors from close, since all we care about is that the channel has been closed
         // at this point.
         self.channel.close(promise: nil)
-        // `closeFuture` is never failed, so we can ignore the error
-        try? await self.channel.closeFuture.get()
+        // `closeFuture` should never be failed, so we could ignore the error. However, do an
+        // assertionFailure to guide bad Channel implementations that are incorrectly failing this
+        // future to stop failing it.
+        do {
+            try await self.channel.closeFuture.get()
+        } catch {
+            assertionFailure(
+                """
+                The channel's closeFuture should never be failed, but it was failed with error: \(error).
+                This is an error in the channel's implementation.
+                Refer to `Channel/closeFuture`'s documentation for more information.
+                """
+            )
+        }
 
         return result
     }