ref(eap-outcomes): dedupe by item_type

[MeredithAnya]

Summary

• Partitions seen_items and duplicate_item_count in AggregatedOutcomesBatch by item_type (HashMap<i32, HashSet<ItemDedupKey>> / HashMap<i32, u64>) so dedup is scoped per item type rather than across all types
• Extracts dedup logic into an is_duplicate(&TraceItem) method on OutcomesAggregator, backed by a record_if_duplicate method on AggregatedOutcomesBatch that encapsulates both the set insert and the count increment
• Updates the accepted_outcomes.duplicate_items counter to emit with an item_type tag

Test plan

• cargo test is_duplicate_per_item_type passes
• Verify accepted_outcomes.duplicate_items metric now has item_type tag in staging

🤖 Generated with Claude Code

[linear-code]

EAP-484 Deduplicate outcomes by item_type in Snuba

[cursor]

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