FINERACT-1659: Prevent duplicate savings interest posting in same period

[sidshas03]

Description

This PR fixes the duplicate interest posting issue reported in FINERACT-1659, where the Post Interest job could post interest multiple times for the same savings account within the same posting period.

What changed

• Added a safeguard in SavingsSchedularInterestPoster to skip accounts where interest has already been posted for the current period, by comparing interestPostedTillDate with the current business date.
• Updated the transaction isolation level to SERIALIZABLE to prevent duplicate postings caused by race conditions during parallel scheduler runs.

Tests

Added two integration tests to validate the behavior:

• testPostInterestPreventsDuplicateOnSameDay
  Ensures running the job twice on the same day results in only one interest posting.
• testPostInterestSkipsCurrentPeriodButAllowsNewPeriod
  Ensures the job skips reposting within the current period, while still allowing postings in a new period.

Expected outcome

• Running the Post Interest job multiple times within the same posting period will not create duplicate transactions, and the savings account balance remains unchanged after subsequent runs.