fix(federation): fix lifting selections when flattening inline fragments

[goto-bus-stop]

Some of our customers have a query shape like this:

# type MySpecialNode implements Node & HasNodes
{
  nodes {
    __typename
    ... on MySpecialNode {
      value
    }
    ... on HasNodes {
      ... on Node {
        __typename
        ... on MySpecialNode {
          value
        }
      }
    }
  }
}

Here, we know that the second MySpecialNode fragment is redundant, as the first instance of the fragment will always already select the nodes.value field. We currently do not remove the redundant fragment.

This PR fixes flatten_unnecessary_fragments so it matches JS QP and removes the second instance of the fragment. The actual fix is 7e9adbd, the other changes in simplify.rs are to add a test and to remove some unnecessary clones.

[github-actions]

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[router-perf]

CI performance tests

• connectors-const - Connectors stress test that runs with a constant number of users
• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload

[goto-bus-stop]

This was the cause: when lifting selections, we were looking at the un-flattened original selection set, instead of the flattened one.

[TylerBloom]

LGTM! Also, I like the clean up work 😄

[duckki]

I see a reduction of mismatches from various graphs. Nice!