test: main fails TestCSVRealPurchase_FailedPurchaseCarriesError after #2238 + #2230; and SP-only runs print the RI hint #2242

Two follow-ups to #2238 (issue #2157):

1. Main is red. fix(cmd): exit non-zero and count Savings Plan failures after a purchase run #2238 makes a real purchase run with any failed purchase return errPurchasesFailed, so the CLI exits non-zero. fix(csv): stop --input-csv replays re-buying ElastiCache and cover the real-purchase path #2230 merged alongside it and added TestCSVRealPurchase_FailedPurchaseCarriesError, which still requires runCSVPurchases to return nil when a purchase fails. So go test ./cmd/ fails on main at 4ed7248.
2. CodeRabbit's finding on fix(cmd): exit non-zero and count Savings Plan failures after a purchase run #2238. A run where only Savings Plans succeeded still prints "Allow up to 15 minutes for RIs to appear in your account". The fix was pushed after fix(cmd): exit non-zero and count Savings Plan failures after a purchase run #2238 merged, so it isn't in main.

🤖 Generated with Claude Code