Compare PT refund CaseData fields by value and fix mislabeled error

[StefanKert]

Summary

Full refunds in QueuePT were rejected with Validation error []: EEEE_Full refund does not match the original invoice '<ref>'. ... (Field: cbCustomer) whenever the receipt's ftChargeItemCaseData (or ftReceiptCaseData / ftPayItemCaseData) was a non-null value such as "" on both sides.

Root cause

ftReceiptCaseData, ftChargeItemCaseData, and ftPayItemCaseData are typed as System.Object on the v2 ReceiptRequest, ChargeItem, and PayItem. RefundValidator compared them with !=, which on object is reference equality. After JSON deserialization both sides are distinct JsonElement instances, so equal values (e.g. "" on the stored original and "" on the incoming refund) always compared as different and the refund failed.

The check inside CompareChargeItems additionally reported the failure as (Field: cbCustomer) — a copy-paste of the cbCustomer mismatch label one method above — which made the error look like a customer-object problem rather than a CaseData problem.

Existing tests didn't catch this because none of them set the *CaseData fields, so both sides were null and null != null is false.

Fix

• Add a CaseDataEquals(object?, object?) helper that treats null and "" as equivalent and otherwise compares via JsonSerializer.Serialize.
• Replace the three reference comparisons in CompareReceiptRequest, CompareChargeItems, and ComparePayItems with CaseDataEquals.
• Correct the Mismatch("cbCustomer") label inside CompareChargeItems to Mismatch("ftChargeItemCaseData").

Test plan

• Re-run the failing scenario: refund a B2C invoice where the original was persisted with "ftChargeItemCaseData": "" and the refund payload also sends "". Validation should now pass.
• Existing PT refund acceptance/unit tests still pass (fiskaltrust.Middleware.Localization.QueuePT.AcceptanceTest, fiskaltrust.Middleware.Localization.v2.UnitTest).
• Spot check: a refund whose ftChargeItemCaseData actually differs from the original still fails, and the error message now reads (Field: ftChargeItemCaseData) rather than (Field: cbCustomer).

Follow-ups (not in this PR)

• Consider replacing the JsonSerializer.Serialize string compare with JsonNode.DeepEquals so structurally-equal but differently-ordered objects also compare equal.
• The same object-typed *CaseData fields exist in QueueES's RefundValidator — worth a follow-up audit there.
• Decide whether the "" ≡ null leniency should be policy across the codebase or scoped to refund validation only.

🤖 Generated with Claude Code

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Queue Test Results

   43 files     43 suites   2m 52s ⏱️
  750 tests   747 ✅ 3 💤 0 ❌
1 876 runs  1 870 ✅ 6 💤 0 ❌

Results for commit 86f73e8.