test: timing-sensitive Go tests fail under -race load #1825

Found during QA of #1824. Under sharded go test -race on 4 cores, TestPerf_GetStatus_AssertThreshold and TestPerf_GetStatus_Parallel_AssertThreshold (handlers) failed; TestCombinedOutputLimited, TestSecureCommandExecutor_ExecuteWithEnv_OutputLimit and TestLogin_AccountWithoutStoredPasswordRunsFullCostCheck flaked earlier. All pass in isolation. Severity: low. Approach: replace wall-clock assertions with event-based checks or load-independent budgets. Note #1824 already loosened the OutputLimit bound in handlers (see #1701).