[v9.2.x] Expressions: Fixes the issue showing expressions editor (#62626)

* 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

(cherry picked from commit 91221bc436)

* unit text fix

pkg/services/query/query_test.go

@@ -524,7 +524,7 @@ func (c *fakePluginClient) QueryData(ctx context.Context, req *backend.QueryData
 	c.req = req
 
 	// If an expression query ends up getting directly queried, we want it to return an error in our test.
-	if req.PluginContext.PluginID == "__expr__" {
+	if req.PluginContext.PluginID == expr.DatasourceUID {
 		return nil, errors.New("cant query an expression datasource")
 	}