Prevent existing participant from adding a participant while executing a retryable statement in a retryable internal txn: In this case, TransactionRouter will call uassert() with ErrorCodes::IllegalOperation.

TransactionRouter::Router::attachTxnFieldsIfNeeded() is where a router adds a new participant. This code is common to routers on mongos and on shards (mongod); thus we should only prevent the creation of a new participant when the opCtx indicates that the caller is an active transaction participant and that it is in a transaction: opCtx->isActiveTransactionParticipant() && opCtx->inMultiDocumentTransaction(). Additionally, opCtx->isRetryableWrite() indicates whether the current context is a retryable write.