B737-800 Flight Attendant reported; that while waiting for the seat belt sign to extinguish; the gate agent attempted to open entry door without an 'ok to open' hand signal. Escape slide did not deploy.

Narrative

I am FA A with 7.5 years seniority. QUICK Taxi in to ZZZ gate. It's dark outside; time being XA:52. The plane is waiting to get to a gate. Plane is stopped-- (not abrupt enough stop to feel like we're at the gate). Seatbelt sign is on. Cabin crew doesn't know we're at the gate. Next; FA B and FA A have knocking on the door from gate agent. Seatbelt sign is still on. FA B and A are confused. Multiple knocks from gate agent. Seatbelt sign is still on. Agent looks through the window and FA B and FA A say NO!" While shaking our heads. Gate agent acts impatient throwing hands up in the air and talking to other people in the jet bridge. Gate agent knocks more. FA A fears agent is going to open the door. FA B and FA A are holding down the forward entry door handle. FA A called the captain and asked if captain forgot to extinguish the seatbelt sign. FA A asks captain if we're parked properly at the gate. FA tells captain the agents are trying to open the door. Captain says "huh oh yes yes yes. And I'll 're-cycle' the seatbelt sign". FA A goes out of SOPs to make the PA and all-call to verify and disarm doors. While the seatbelt sign is still on; FA A gives the okay to open the forward entry door. Seatbelt sign is still on. FA A calls the captain again and says; "the seatbelt sign is still on; I think it's broken". Captain says; 'well we had the engine still cooling down.' Later; Captain tells FA A that he couldn't hear FA A well on the intercom because he had taken his headset off already and the radio sounded so bad. Captain was in high workload scenario because he was going through his checklist as well.Suggestions: -gate agents need to be more patient and fear the door more.-gate agents based at quick taxi stations need to be aware that flight/cabin crew need time for our procedures to cool engines; turn off engines; extinguish seatbelt signs; verify and disarm doors; all call; and cross checks. -pilots need to communicate its going to be a longer sit at the gate while we sit with the engines on.-pilots need to not slack on communication between cabins.-FAs need to know when we will be sitting at the gate for longer than usual with the engines still running.-I recommend the FA B on probation NOT get in any trouble for this very unusual circumstance. I; FA A; at 7.5 years seniority completely took over the situation. FA a feared the agent opening the door while it was armed. FA A is the one who initiated all call out of SOPs before the seatbelt sign was extinguished. FA A got the okay from the captain First. The engines were NOT on. FA B and FA A both told the agent 'NO' while shaking our heads and FA A took further measures to prevent an Inadvertent Slide Deployment (ISD)."