[GraphQL/Limits] BUGFIX: directives test error type (#18661)

## Description

Passing an unsupported directive should be a user input error, not an
internal error.

## Test plan

```
sui-graphql-e2e-tests$ cargo nextest run --features pg_integration -- limits/directives
```

## Stack
- #18660

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [x] GraphQL: Passing an unsupported directive to the service will be
treated as a `BAD_USER_INPUT` rather than an `INTERNAL_SERVER_ERROR`.
- [ ] CLI: 
- [ ] Rust SDK:

crates/sui-graphql-e2e-tests/tests/limits/directives.exp

@@ -16,7 +16,7 @@ Response: {
         }
       ],
       "extensions": {
-        "code": "INTERNAL_SERVER_ERROR"
+        "code": "BAD_USER_INPUT"
       }
     }
   ]
@@ -35,7 +35,7 @@ Response: {
         }
       ],
       "extensions": {
-        "code": "INTERNAL_SERVER_ERROR"
+        "code": "BAD_USER_INPUT"
       }
     }
   ]

crates/sui-graphql-rpc/src/extensions/query_limits_checker.rs

@@ -408,7 +408,7 @@ fn check_directives(directives: &[Positioned<Directive>]) -> ServerResult<()> {
     for directive in directives {
         if !allowed_directives().contains(&directive.node.name.node.as_str()) {
             return Err(graphql_error_at_pos(
-                code::INTERNAL_SERVER_ERROR,
+                code::BAD_USER_INPUT,
                 format!(
                     "Directive `@{}` is not supported. Supported directives are {}",
                     directive.node.name.node,