test: add comprehensive tests for InsightsService month period logic#1

oronno wants to merge 2 commits intomainoronno/privateledger:mainfrom

claude/write-test-mk7erq5j2s0zl4zp-38xFGoronno/privateledger:claude/write-test-mk7erq5j2s0zl4zp-38xFGCopy head branch name to clipboard