feat: add 10 diverse fixture items for agent unit testing

- 5 simple items (card_swallowed, change_pin, terminate_account,
  top_up_by_cash_or_cheque, lost_or_stolen_card)
- 5 ambiguous items on known confusion boundaries
  (unable_to_verify_identity, Refund_not_showing_up, pending_transfer,
  card_payment_not_recognised, why_verify_identity)
- Each has query_id, text, true_intent, difficulty
- All query_ids verified against holdout.parquet

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: saviolobo

tests/fixtures/items/item_001.json

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+{
+  "query_id": "test_01938",
+  "text": "What do I do if the ATM ate my card?",
+  "true_intent": "card_swallowed",
+  "difficulty": "simple"
+}

tests/fixtures/items/item_002.json

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+{
+  "query_id": "test_02145",
+  "text": "I need to change my PIN.",
+  "true_intent": "change_pin",
+  "difficulty": "simple"
+}

tests/fixtures/items/item_003.json

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+{
+  "query_id": "test_01901",
+  "text": "Can you delete my account please?",
+  "true_intent": "terminate_account",
+  "difficulty": "simple"
+}

tests/fixtures/items/item_004.json

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+{
+  "query_id": "test_02441",
+  "text": "How do I top-up using cash?",
+  "true_intent": "top_up_by_cash_or_cheque",
+  "difficulty": "simple"
+}

tests/fixtures/items/item_005.json

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+{
+  "query_id": "test_00468",
+  "text": "My card got lost.",
+  "true_intent": "lost_or_stolen_card",
+  "difficulty": "simple"
+}

tests/fixtures/items/item_006.json

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+{
+  "query_id": "test_01203",
+  "text": "I am having trouble verifying my identity.",
+  "true_intent": "unable_to_verify_identity",
+  "difficulty": "ambiguous"
+}

tests/fixtures/items/item_007.json

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+{
+  "query_id": "test_01784",
+  "text": "I requested a refund from a store but it hasn't arrived.",
+  "true_intent": "Refund_not_showing_up",
+  "difficulty": "ambiguous"
+}

tests/fixtures/items/item_008.json

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+{
+  "query_id": "test_01879",
+  "text": "How long does it take for a money transfer to show?",
+  "true_intent": "pending_transfer",
+  "difficulty": "ambiguous"
+}

tests/fixtures/items/item_009.json

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+{
+  "query_id": "test_01110",
+  "text": "I see a charge on my account that I don't recall making. I feel like my account may have been compromised.",
+  "true_intent": "card_payment_not_recognised",
+  "difficulty": "ambiguous"
+}

tests/fixtures/items/item_010.json

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+{
+  "query_id": "test_01170",
+  "text": "I would like to not have to do the identity verification.",
+  "true_intent": "why_verify_identity",
+  "difficulty": "ambiguous"
+}