fix(federation): keep condition exclusions duplicate-free so equality and cache lookups are set-based

[inanna-apollo]

ExcludedConditions is compared as a set (equal length plus one-way membership), but add_item appended unconditionally. Once a condition was added twice, equality stopped being symmetric ([A, A] == [A, B] but not the reverse), and ConditionResolverCache::contains could return a resolution cached under [A, A] for a request excluding [A, B].

Make add_item a no-op when the condition is already excluded, matching ExcludedDestinations::add_excluded. The length+containment equality is then a correct set equality, so the cache key is independent of how a set of exclusions was built. Current planner and composition call sites already skip edges whose condition is excluded before adding it, so plans do not change.

Reachability

Not reachable from current planner paths: every call site skips edges whose condition is already excluded before adding it (a temporary duplicate-add panic never fired across the full lib and --test main suites). This makes the data structure's equality and the condition cache correct by construction; plans do not change.

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
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• Tests added and passing
  • Unit tests
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  • Manual tests, as necessary

[apollo-librarian]

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

Build ID: 688cdc0ac113668888bdddd6
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URL: https://www.apollographql.com/docs/deploy-preview/688cdc0ac113668888bdddd6

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