fix(demand_control): use correct parent type in fragment definitions

[goto-bus-stop]

Prerequisite for https://github.com/apollographql/router/pull/6013.

Fixes errors like:

Attempted to look up a field on type MyInterface, but the field does not exist

Fragment definitions have a type condition, and all selections inside the fragment are executed against that type condition. Previously, the demand control cost calculator executed them against the parent type where the fragment is used, which is often different.

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• [x] Changes are compatible[^1]
• [ ] Documentation[^2] completed
• [x] Performance impact assessed and acceptable
• Tests added and passing[^3]
  • [x] Unit Tests
  • [ ] Integration Tests
  • [ ] Manual Tests

[^1]: It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. [^2]: Configuration is an important part of many changes. Where applicable please try to document configuration examples. [^3]: Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions.

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[github-actions[bot]]

@goto-bus-stop, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.

[router-perf[bot]]

CI performance tests

• [ ] connectors-const - Connectors stress test that runs with a constant number of users
• [x] 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
• [x] 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]

oops, i had enabled auto-merge. I'll follow up with a changeset