test(service): transaction_history_service.apiBasedSync (+8 tests)

[TaprootFreak]

Summary

Stage 19 of the coverage push. Closes the previously-deferred apiBasedSync gap on TransactionHistoryService — kept in a separate file from `fetchPendingTransactions` (#336) to keep the repository mocks focused on the sync flow.

Method under test	Test file	Cases
`TransactionHistoryService.apiBasedSync`	`test/packages/service/transaction_history_service_sync_test.dart`	8

What it covers

• Non-200 on `/v1/realunit/account//history` short-circuits: no repository writes (the cubit's early return when AccountHistoryDto is null).
• Entries whose `transfer` field is null are skipped.
• Inserts a plain `Transaction` when there is no matching DFX row.
• Inserts a DFX-enriched transaction (`insertDfxTransaction`) when `/v1/transaction` has a row whose `id` is not null and whose `inputTxId` matches the history hash.
• Existing transactions go through `updateTransaction` / `updateDfxTransaction` instead of insert (governed by `existsTransaction`).
• The DFX-row match also accepts `outputTxId` — both directions are pinned.
• DFX rows whose `id` is null fall back to the plain-Transaction insert path.

Notes

• Both endpoints are fetched via `Future.wait`. Reading `request.url.path` lets a single MockClient handler return different bodies for the two paths.
• The test stubs `existsTransaction` to return false by default, then flips it to true for the update-path tests.

Test plan

• `flutter analyze` on the new file — clean
• `flutter test` — 8 / 8 passing locally
• CI green