* Add tests and logic for Simplefin account balance normalization
- Introduced `SimplefinAccountProcessorTest` to verify balance normalization logic.
- Updated `SimplefinAccount::Processor` to invert negative balances for liability accounts (credit cards and loans) while keeping asset balances unchanged.
- Added comments to clarify balance conventions and sign normalization rules.
* Refactor balances-only sync logic and improve tests for edge cases
- Updated `SimplefinItem::Importer` and `SimplefinItem::Syncer` to ensure `last_synced_at` remains nil during balances-only runs, preserving chunked-history behavior for full syncs.
- Introduced additional comments to clarify balances-only implications and syncing logic.
- Added test case in `SimplefinAccountProcessorTest` to verify correct handling of overpayment for credit card liabilities.
- Refined balance normalization in `SimplefinAccount::Processor` to always invert liability balances for consistency.
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Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>
