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

client/src/api/client.ts

@@ -192,8 +192,11 @@ export const scenarios = {
   remove(id: string): Promise<void> {
     return request(`/scenarios/${id}`, { method: 'DELETE' });
   },
-  run(id: string): Promise<ScenarioRun> {
-    return request(`/scenarios/${id}/run`, { method: 'POST' });
+  run(id: string, environmentId: string): Promise<ScenarioRun> {
+    return request(`/scenarios/${id}/run`, {
+      method: 'POST',
+      body: JSON.stringify({ environmentId }),
+    });
   },
   getRun(scenarioId: string, runId: string): Promise<ScenarioRunDetail> {
     return request(`/scenarios/${scenarioId}/run/${runId}`);