Refactor transaction item parsing

[heisenbugged]

Summary

• Add a small TransactionItem value object for the existing stored transaction item format.
• Use it for transaction matching, clearing, display, public event sanitizing, and trade offer rendering.
• Keep saved transaction item serialization exactly unchanged.

Test Plan

• mvn -Dtest=ti4.service.transaction.TransactionItemTest,ti4.helpers.TransactionHelperTest test

[github-actions]

🔬 Test Results

✅ JSON Validation

No JSON files changed.

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✅ Formatting Checks Passed

All files pass Spotless formatting.

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✅ JUnit Tests Passed

All tests passed successfully.

[Fingon00]

I find the flow of this much harder to parse/work with. It will make it more difficult to debug/fix/add to transactions in the future as a result.