GET HTTP queries are not idempotent

[robsimmons]

• EdgeDB Version: 6.2
• EdgeDB CLI Version: 7.0.3
• OS Version: OSX

Steps to Reproduce:

1. Create a local database with this schema

   using extension edgeql_http;
   
   module default {
     type Person {
       required name: str;
     };
   }
   

2. Run the following command several times:

   curl -i -H "Content-Type: application/json" --request GET -u admin:<PASSWORD> "http://localhost:10700/branch/main/edgeql?query=insert%20Person%20%7Bname%3A%3D%20%22Fred%22%7D%3B%0A%0A%0A"
   

   which URLencodes the query

   insert Person {name:= "Fred"};
   

The expected behavior is that this query is rejected, like a GraphQL query containing a mutation would be rejected, given that it changes the database in a non-idempotent manner. The docs say "[Since HTTP is a stateless protocol, no DDL, transaction commands, can be executed using this endpoint.](https://docs.geldata.com/reference/clients/http)", but that applies equally to insertions, deletions, and updates.

The observed behavior is that each GET request adds a Person entity to the database.

[msullivan]

I wonder if we actually disallow GraphQL mutations on GET, too.