fix(query-planner): restore missing defer dependencies after transitive reduction

[duckki]

Background

When the query planner builds a fetch dependency graph for queries with @defer, it performs a transitive reduction to eliminate redundant edges. However, when a direct edge from an ancestor fetch to a deferred fetch node is also reachable transitively through an intermediate fetch, the reduction removes the direct edge. This causes extract_children_and_deferred_dependencies to miss registering the ancestor as a defer dependency, so at runtime the deferred node never receives the ancestor's broadcast data (__typename, entity keys, and other fields at that response path). The deferred fetch is then silently skipped, returning null for fields that should have data.

Proposal

The fix tracks defer-crossing edges that are removed during transitive reduction in a new reduced_defer_edges field on FetchDependencyGraph. After reduction, collect_reduced_defer_dependencies examines each removed edge: it navigates the source fetch's selection set down to the target's merge_at level using selection_set_at_path, then checks whether the source provides any fields the deferred target's inputs require (using SelectionMap::get() with SelectionKey for type-condition-aware matching). Only edges where a real field intersection exists are restored as dependencies, avoiding unnecessary over-registration.

Testing

The change includes two new federation-level snapshot tests—one for the original reproducer where the intermediate fetch has a deeper merge_at path, and one where the intermediate has the same merge_at as the deferred target—plus a router-level integration test that exercises the full defer execution pipeline and verifies the deferred response is not silently dropped.

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Checklist

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• PR description explains the motivation for the change and relevant context for reviewing
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• Changeset is included for user-facing changes
• Changes are compatible[^1]
• Documentation[^2] completed
• Performance impact assessed and acceptable
• Metrics and logs are added[^3] and documented
• Tests added and passing[^4]
  • Unit tests
  • Integration tests
  • Manual tests, as necessary

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