Add retry-resilient regression coverage for MerchantBalanceStateRepository #1869

Problem

MerchantBalanceStateRepository hits SQL Server directly and Sentry issue 1841 shows a transient failure shape coming from the read path.

Related investigation: TransactionProcessor#1841

Requested change

Add a targeted regression test and follow-up work for the retry-resilient merchant balance read path.

Suggested coverage

• verify the balance load path handles transient SQL failures consistently
• verify the repository or its wrapper surfaces a useful failure if the database throws
• if retry configuration is introduced, confirm this path uses it

Notes

• Keep the first change focused on the merchant balance state repository.
• If the fix belongs in shared EF configuration, link it back there as well.

Acceptance criteria

• A regression test exists for the merchant balance load path
• The failure/retry behavior is explicit
• The issue is linked to the shared retry work if the fix spans repos