Create factory for PendingTransactionStorage selection

[konstantinabl]

Problem

Storage selection logic lives in Relay:

const storage = this.redisClient
  ? new RedisPendingTransactionStorage(this.redisClient)
  : new LocalPendingTransactionStorage();

Relay shouldn't know about storage implementation details.

Solution

Add factory method:

export class PendingTransactionStorageFactory {
  static create(redisClient?: RedisClientType): PendingTransactionStorage {
    return redisClient 
      ? new RedisPendingTransactionStorage(redisClient)
      : new LocalPendingTransactionStorage();
  }
}

// In Relay
const storage = PendingTransactionStorageFactory.create(this.redisClient);

Benefits

• Encapsulates storage selection logic where it belongs
• Relay stays focused on orchestration
• Easier to add new storage types

Acceptance Criteria

• Factory method created
• Relay uses factory
• Tests passing