GraphQL G2: client HTTP binding, errors and local validation #1584

Part of #1581. Depends on G0.

GraphqlClientAdapter over a borrowed org.restlet.Client:

• POST with Accept: application/graphql-response+json, application/json;q=0.9; variables serialized as JSON values, never templated as text.
• Documents as String or graphql.language.Document (printed with AstPrinter).
• Optional setSchema(GraphQLSchema): parse and validate locally, return errors without a network call. The extension never fetches a schema itself.
• A 4xx with a GraphQL body becomes an ExecutionResult with errors; connection failures, timeouts and non-GraphQL responses throw GraphqlTransportException with the status and a bounded body excerpt.
• maxResponseBytes (10 MiB) enforced, never truncated silently.
• start()/stop() lifecycle; the borrowed Client is never stopped. Request customizer for API-key headers.
• Round trip against the G1 server and against a third-party GraphQL server in test scope.