Expressions: Fixes the issue showing expressions editor (#62510)

* Use suggested value for uid

* update the snapshot

* use __expr__

* replace all -100 with __expr__

* update snapshot

* more changes

* revert redundant change

* Use expr.DatasourceUID where it's possible

* generate files

pkg/services/ngalert/api/test-data/test.http

@@ -15,7 +15,7 @@ content-type: application/json
                 "from": 18000,
                 "to": 10800
             },
-            "datasourceUid": "-100",
+            "datasourceUid": "__expr__",
             "model": {
                 "type":"math",
                 "expression":"1 < 2"
@@ -56,7 +56,7 @@ content-type: application/json
       "from": 18000,
       "to": 10800
     },
-    "datasourceUid": "-100",
+    "datasourceUid": "__expr__",
     "model": {
       "expression": "$A",
       "intervalMs": 2000,
@@ -113,7 +113,7 @@ content-type: application/json
                 "from": 18000,
                 "to": 10800
             },
-            "datasourceUid": "-100",
+            "datasourceUid": "__expr__",
             "model": {
                 "type":"math",
                 "expression":"1 < 2"
@@ -121,7 +121,7 @@ content-type: application/json
         }
     ]
   }
-}}
+}
 
 ### grafana recipient - lotex payload
 POST http://admin:admin@localhost:3000/api/v1/rule/test/{{grafanaRecipient}}
@@ -129,4 +129,4 @@ content-type: application/json
 
 {
   "expr": "rate({cluster=\"us-central1\", job=\"loki-prod/loki-canary\"}[1m]) > 0"
-}
+}