DEV: Improve validity checking of JSON param values. (#35401)

The validity checking for parameter values being passed in expects them
all to be strings, then casts them to their appropriate type after
confirming that they're valid.

Rather than bypassing these checks, it's safer to convert the values to
strings, then continue with the existing validity checks.

plugins/discourse-data-explorer/lib/discourse_data_explorer/parameter.rb

@@ -110,6 +110,9 @@ module DiscourseDataExplorer
 
     def cast_to_ruby(string)
       string = @default unless string
+      # Since we allow passing a JSON object for the params, the most straightforward way to
+      # check that the value is valid is to convert it back to a string.
+      string = string.to_s
 
       if string.blank?
         if @nullable

plugins/discourse-data-explorer/spec/requests/query_controller_spec.rb

@@ -199,7 +199,8 @@ describe DiscourseDataExplorer::QueryController do
                  foo: 42,
                  bar: "test",
                },
-             }
+             },
+             as: :json
         expect(response.status).to eq(200)
         expect(response_json["success"]).to eq(true)
         expect(response_json["columns"]).to eq(%w[my_value text_value])