fix(bigquery/storage/managedwriter): faster context failure on send (#10169)

This is a minor PR that accelerates the failure loop for context expiration cases.  Previously, the connection abstraction would deal with expired context on the receiver side, and this change simply checks context before allowing a new append to succeed.

Tests already existed that exhibited this behavior, but were timing dependent and masked the issue (the receiver would close the connection fast enough).

Fixes: https://togithub.com/googleapis/google-cloud-go/issues/10128

Author: shollyman

bigquery/storage/managedwriter/connection.go

@@ -369,6 +369,11 @@ func (co *connection) lockingAppend(pw *pendingWrite) error {
 		}
 	}()
 
+	// If connection context is expired, error.
+	if err := co.ctx.Err(); err != nil {
+		return err
+	}
+
 	var arc *storagepb.BigQueryWrite_AppendRowsClient
 	var ch chan *pendingWrite
 	var err error