fix: avoid deep-hashing the response subtree during reference extraction

[sethrperkins]

extract_enums_from_selection_set dedupes visited fragment spreads with a
HashSet<(&str, &Object)>. Hashing &Object hashes the pointed-to value, so
every fragment spread deep-hashes the whole response subtree. With
metrics_reference_mode: extended, extraction cost scales with response size
times fragment-spread count, and operations with many fragments over large
responses regress noticeably.

Keying on the object's pointer (*const Object) dedupes on identity, which is
all this needs, and restores O(1) insertion.

Introduced in #9473.

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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 compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Metrics and logs are added³ and documented
• Tests added and passing⁴
  • Unit tests
  • Integration tests
  • Manual tests, as necessary

Exceptions

• Documentation, metrics, and logs: no impact. This is an internal fix with no configuration or behavior change.
• Tests: the existing apollo_studio_interop unit tests exercise fragment-spread deduplication and pass unchanged. The change only swaps the dedup key from value to identity and preserves output, so no new test was added.

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