[devin-ai-integration]

Summary

Stacked on #332. A transfer that would leave the sender at exactly $0 is now rejected with Insufficient funds.

 // AccountService.transferAmount
-if (fromAccount.getBalance().compareTo(amount) < 0) {
+if (fromAccount.getBalance().compareTo(amount) <= 0) {
     throw new RuntimeException("Insufficient funds");

Only the transfer guard changes. Withdrawals (withdraw) still allow taking out the full balance.

This change conflicts with the plan. MT-040 in qa/manual-test-plan.csv says "Customer can transfer the full balance" and expects the sender at $0.00 and the recipient at $100.00. This PR leaves the plan and the MT-040 test as they are. If this rule is intended, someone needs to update MT-040's expected result. Locally against MySQL 8, MT-040 is the only row that fails (53 passed, 1 failed). The e2e check on this PR is expected to go red on MT-040.

Link to Devin session: https://devin-gtm.devinenterprise.com/sessions/4e5ce8a64e094b30882fb37f4f9983c5
Open in Devin Desktop: https://devin-gtm.devinenterprise.com/desktop/session/4e5ce8a64e094b30882fb37f4f9983c5?variant=devin
Requested by: @GarretAnd

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[devin-ai-integration]

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