fix(txmgr): prevent resource leak from unstopped tickers in sendTx

[maradini77]

The sendTx method in SimpleTxManager was creating new time.Ticker instances on each loop iteration without properly stopping previous ones. This caused a resource leak where:

• Background goroutines from tickers accumulated over time
• Timer resources were never released
• Under high transaction volume, this could lead to memory exhaustion and degraded performance