* Add tests and update logic for processing SimpleFIN investment transactions
- Added `SimplefinAccount::Transactions::ProcessorInvestmentTest` to validate dividend transaction processing, transaction linking, and stale linkage repairs.
- Enhanced `SimplefinItem#process_accounts` with stale linkage repair logic and detailed logging for unlinked accounts with transactions.
- Updated `SimplefinAccount::Transactions::Processor` for improved logging and error handling during transaction processing.
- Adjusted `SimplefinItem::Importer` to log detailed account and transaction information and use extended sync windows for investment accounts.
* Refactor `SimplefinItem#process_accounts` to use direct queries for fresh data and streamline stale linkage repair logic; update tests for improved coverage and clarity.
* Improve stale linkage repair logic in `SimplefinItem#repair_stale_linkages`
- Updated to handle multiple linked accounts matching the same unlinked account by selecting the first match.
- Added detailed logging to warn about multiple matches for easier debugging.
* Include `:linked_account` in `SimplefinItem#process_accounts` queries for more comprehensive account data processing.
* Expand `merge_transactions` logic with composite key fallback for deduplication; document edge cases.
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Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>
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