refactor(session): auto-create named browser sessions

- create and register named browser sessions on open/exec when missing

- align scenario step DTO/entity/service by removing sessionName usage

- update MCP scenario-step schemas to use optional title fields

server/src/scenario/scenario-step.entity.ts

@@ -29,9 +29,6 @@ export class ScenarioStepEntity {
   @Column({ type: "text", nullable: true })
   title: string | null;
 
-  @Column({ type: "text", nullable: true })
-  sessionName: string | null;
-
   @Column({ type: "text", nullable: true })
   execCode: string | null;