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/dto/create-scenario-step.dto.ts

@@ -7,16 +7,6 @@ export class CreateScenarioStepDto {
   @IsString()
   title?: string;
 
-  @ApiPropertyOptional({
-    description:
-      "Session name (deprecated — browser is created automatically per run).",
-    example: "my-session",
-  })
-  @IsOptional()
-  @IsString()
-  @IsNotEmpty()
-  sessionName?: string;
-
   @ApiPropertyOptional({ example: "return await page.title();" })
   @IsOptional()
   @IsString()