Simplify sort-clause de-duplication and document single-key invariant

[jwils]

What

DatastoreQuery#remove_duplicate_sort_clauses used:

sort_clauses.select do |clause|
  clause.keys.all? { |key| seen_fields.add?(key) }
end

This relied on Set#add?'s side effect inside all?, conflating filtering with mutation. For a multi-key clause, all? short-circuits as soon as a previously-seen key is encountered, leaving the remaining keys of that clause unrecorded in seen_fields — a subtle correctness bug.

In practice this branch is unreachable: sort clauses are always single-key, enforced upstream by DecodedCursor::Factory.from_sort_list, which raises Errors::InvalidSortFieldsError ("Each must be a flat hash with one entry") for anything else.

Changes

• Simplified the method to operate directly on the single key via seen_fields.add?(clause.keys.first).
• Added a comment documenting the single-key invariant and why add? correctly drops duplicates.

This follows the project guideline to avoid defensive code for impossible cases rather than adding multi-key handling for a scenario that can't occur.

Testing

bundle exec rspec elasticgraph-graphql/spec/unit/elastic_graph/graphql/datastore_query/sorting_spec.rb \
                  elasticgraph-graphql/spec/unit/elastic_graph/graphql/datastore_query/merge_spec.rb
# 45 examples, 0 failures