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/browser/dto/exec.dto.ts

@@ -4,7 +4,7 @@ import { IsOptional, IsString, IsUrl } from "class-validator";
 export class ExecDto {
   @ApiPropertyOptional({
     description:
-      "Session name previously created by POST /login. If omitted, executes without a stored session.",
+      "Optional named browser session. If it does not exist yet, it is created automatically.",
     example: "test-session-1",
   })
   @IsOptional()