feat(scenarios): require environment for scenario runs

- require environmentId when creating runs in API and MCP tools

- pass selected environment data into step execution helpers

- prompt for environment selection before running scenarios in UI

server/src/scenario/dto/create-scenario-run.dto.ts

@@ -0,0 +1,7 @@
+import { IsNotEmpty, IsUUID } from "class-validator";
+
+export class CreateScenarioRunDto {
+  @IsUUID()
+  @IsNotEmpty()
+  environmentId: string;
+}