OXEAN-133 Remove generated relation filter syntax

[GiuseppeXD]

Description ✍️

Remove relation-filter syntax from Graphoid's generated GraphQL schema. Nested relation predicates previously execute recursively and can materialize an unbounded related-record set.

Overview 🔍

Generated model filters now expose only scalar fields, scalar operators, OR, and AND.

# Rejected by static schema validation
accounts(where: { labels_some: { id_not: null } }) { id }
people(where: { account: { id_not: null } }) { id }

# Still valid
accounts(where: { stringField: "account" }) {
  labels(order: { id: ASC }, limit: 1, skip: 0) { id }
}

Generated to-many relation resolvers no longer expose where; order, limit, and skip remain available.

Important

This is a breaking GraphQL schema change. Clients must filter relation data with scalar parent fields or separate root queries.

Checks ☑️

• relation_filter_restrictions_spec.rb: 6 examples, 0 failures
• Ruby syntax checks pass for modified source and spec files
• git diff --check passes