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/i18n/locales/en.json

@@ -145,7 +145,11 @@
     "cred_form_cred_required": "Select a credential",
     "cred_form_alias": "Alias",
     "cred_form_alias_placeholder": "e.g. api_key",
-    "cred_form_alias_required": "Alias is required"
+    "cred_form_alias_required": "Alias is required",
+    "run_modal_title": "Select environment",
+    "run_modal_env_label": "Environment",
+    "run_modal_no_env": "No environments found. Create an environment before running a scenario.",
+    "run_modal_env_required": "Please select an environment"
   },
   "theme": {
     "switch_to_light": "Switch to light theme",