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.service.ts

@@ -120,7 +120,6 @@ export class ScenarioService {
         order: currentCount,
         scenarioId,
         title: dto.title ?? null,
-        sessionName: dto.sessionName ?? null,
         execCode: dto.execCode ?? null,
         validateCode: dto.validateCode ?? null,
       }),
@@ -153,7 +152,6 @@ export class ScenarioService {
       ...dto,
       order: step.order,
       title: dto.title ?? step.title,
-      sessionName: dto.sessionName ?? step.sessionName,
       execCode: dto.execCode ?? step.execCode,
       validateCode: dto.validateCode ?? step.validateCode,
     });