Stabilize nested relationship request assertions

[jwils]

Why

Nested relationship acceptance tests assumed that each logical search would use a separate _msearch transport request. Fiber scheduling can batch the same six searches into five requests, causing unrelated changes such as #1316 to fail despite correct results and routing.

What

• Stop asserting the transport request count in the bi-directional nested relationship test
• Continue asserting six logical datastore searches and their exact shard routing

Risk Assessment

Low — this is a test-only change that preserves the result, query-count, and routing coverage.

References

• Reproduced in the Ruby 3.4 and 4.0 run_each_gem_spec jobs on Provide path_to_schema to schema definition state #1316
• bundle exec rspec elasticgraph-graphql/spec/acceptance/nested_relationships_spec.rb
• script/lint