fix(federation): merge fragments whose keys collide after directive stripping

[inanna-apollo]

remove_unneeded_top_level_fragment_directives removes @include/@skip applications from top-level inline fragments when they are already implied by the path a fetch node is merged at. Removing a directive changes the fragment's selection key, so two fragments that were distinct before (... on A @include(if: $x) { id } and ... on A { value }) can end up with the same key. The result was built with SelectionMap::insert, which appends without checking for an existing key, so the returned selection set held two entries for one key, breaking the invariant every key-based lookup and merge on SelectionMap relies on.

Build the result with add_local_selection, which merges selections whose keys collide.

Reachability

Not reached by the planner today: instrumenting every strip across the query plan suite and hand-built shapes found no collision, and the only caller re-merges top-level duplicates. Defensive fix for the SelectionMap key invariant.

Testing

The regression test(s) in this PR fail on dev and pass with this change; neighboring test suites pass with no snapshot changes. Found during property-based testing of apollo-federation.

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Checklist

• PR description explains the motivation for the change and relevant context for reviewing
• PR description links appropriate GitHub/Jira tickets (creating when necessary)
• Changeset is included for user-facing changes
• Changes are compatible
• Documentation completed
• Performance impact assessed and acceptable
• Metrics and logs are added and documented
• Tests added and passing
  • Unit tests
  • Integration tests
  • Manual tests, as necessary

[apollo-librarian]

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File Changes

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* graphos/routing/(latest)/_sidebar.yaml

Build ID: 02cf6155759e9914b4639c6e
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URL: https://www.apollographql.com/docs/deploy-preview/02cf6155759e9914b4639c6e

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